Players make many types of mistakes. Today I'll break down the genres of mistakes, give an example or two, and suggest ways to avoid them.
Most mistakes fall into one or more of seven categories: glaring misplays, autopilot errors, gameplan mistakes, sequencing errors, timing issues, misevaluations, and extracurricular mistakes. Each has varying levels of subtlety, and the extent to which you get punished for committing them is more or less dependent on context.
Let's get the big one out of the way first. This is when you make a play that is strictly wrong—for example, tapping out, then attacking your 2/2 into their 3/3 when your creature dying has no benefit to you. All you accomplished was a loss of material and a blow to your self-esteem.
My favorite personal example was when I was in a huge board stall with neither player able to break through. Eventually my opponent cast a Lure and attacked with all of their creatures... except the creature enchanted by the Lure. That was clearly a misclick, but a glaring misplay nonetheless. Usually these mistakes aren't a failure in strategic positioning, but a failure in the mechanics of playing the game. Hopefully, since you are reading this, you don't do this very often, but it still happens to everyone. Sometimes these mistakes are less obvious than other times, but they're never less painful.
The only real way to prevent glaring misplays from happening is to focus. That's it. The simplest mistakes have the simplest solutions. If you have a method to prevent yourself from making these mistakes, use them. Some people put a marker on their deck to remember upkeep triggers. Some people write down every card in their opponent's hand after a Thoughtseize. These might appear "scrubby" to some, but don't be ashamed of the small things that help you focus. There is so much going on in any given game that taking some of the pressure off of your brain's bandwidth is valuable.
The type of mistake that I make more than any other. Autopilot errors occur when you think you've played enough of a format or a matchup that you go into autopilot. When 95% of the time a certain line of play is the right thing to do, you usually just do it and it goes well, but sometimes you can sniff out that perhaps your opponent's holding up a Counterspell, or that they don't have a Wrath of God effect. Heuristics, like Lightning Bolt the Birds of Paradise, perpetuate these mistakes. Sometimes the texture of the game will make you want to take a different line of play than heuristics dictate, and those spots are where you can get a leg up on an opponent playing bread-and-butter Magic.
This particular autopilot error comes from a match I played recently. Typically, you're supposed to cast Daybreak Charger after playing another creature in order to get the value from its enters-the-battlefield trigger. Since I was only half-paying attention, I just cast Wildwood Scourge on turn two because I didn't have a third land, which was awful for many reasons.
If I play Daybreak Charger on turn two instead, I attack for the same amount as I do if I curve Wildwood Scourge into Daybreak Charger. On top of that, If I do draw a land, I can make Wildwood Scourge a 2/2. I also probably want to prioritize keeping Wildwood Scourge alive. All of this would lead me to playing Daybreak Charger first if I'd thought for a couple seconds on it instead of playing on autopilot. (In my defense I was probably playing Monster Train during the match.)
Pay attention to what your opponent is doing every turn of the game, from the order they play their lands to what removal spell they play on which creature. It's all information you can use to deduce what your opponent is thinking and what cards they're holding. Don't waste the information your opponent is handing you. At the same time, be open to unorthodox lines of play, even if you don't end up making them; no matter the outcome of the game, you'll at least know you didn't miss out on something you didn't consider.
Gameplan mistakes, or misassignment of role, come about because making a fundamentally correct play can be contradictory to your game plan and vice-versa. Lets say you're playing an aggressive deck. You have four 2/2 creatures in play and your opponent is tapped out and has a 3/3. You should probably be "chump attacking" here more often than you think; that creature you lose is probably worth sacrificing for 6 damage. It is important that you figure out both what it takes for you to win a given matchup and what has to go wrong for you to lose. Once you've done that, every play should be made with your role in mind. Don't be the control player who attacks their Fetid Imp into a red mage packing haste creatures, and don't be the aggressive player looking to let off the gas pedal.
Almost everyone has already seen the Lightning Helix heard 'round the world, but it is a great example of having a game plan. The only way Craig Jones was going to win the game was to try to burn Olivier Ruel out with a topdecked burn spell. Had Jones killed the Hand of Cruelty, he would have almost certainly lost, and that famous topdeck would've never existed.
Sequencing errors are when you play things in the wrong order. This one comes up most often during the early turns of the game, in mana development. Playing an Overgrown Tomb tapped on turn one with a Temple of Malady in hand is a sequencing error, but you can also play your threats or your answers out of order. For example, on turn three, if you cast a two-drop instead of a three-drop, then you can't double spell on turn four.
Yam Wing Chun saw the win, he just needed to attack. Sadly for him, he forgot that he needed to discard first, so when it became time to attack, Hazoret the Fervent couldn't. That's an extreme, game-ending example, but sequencing errors are often glaring misplays' quiet cousin. You know you messed up, but your opponent doesn't.
To prevent this from happening, try to have a plan for all the cards in your hand every time you play something, even your first land. Gabriel Nassif is known for taking longer than most for every action, including which tapped land to play on turn one, but taking the time to think will help you figure out if the card you are playing now may be better suited for the future, which will illuminate the right line to of play take in the present.
I could probably write a whole article about timing issues. Timing issues are when you make your game actions in the wrong order. For example, playing your creature pre-combat, then not being able to represent a combat trick when you attack. These mistakes are usually minute, but they give away information that will be valuable to your opponent long term. Optimal timing involves toeing the line between giving your opponent as little information as you can while also giving them as few options as possible, while simultaneously doing the opposite for yourself. If you wait too long, you give them more options to play around your card, be it with more mana available or more cards in their hand.
Ciaran Mackenzie waited until the last possible moment to cast Galvanic Blast, but in this spot, waiting that long gave his opponent more options. If your opponent is tapped out, it is usually right to cast your removal spell on your turn, even if it's an instant, but if they have mana open, it is a good idea to wait until their turn so that they have to use their mana on their own turn, when they'd otherwise use that mana to progress their own game plan, in order to interact with it.
The best way to avoid these mistakes is to periodically examine the board from your opponent's perspective. When would it be the most difficult for them to interact with your play? Think about what information you are conveying to them by the timing of your spells. Remember, your job is to make your opponent's decisions as difficult as possible.
Misevaluations are when you think a card is more or less important in a matchup than it is. If your only answer to Ugin, the Spirit Dragon is that Murderous Rider in your hand, it's probably not a good idea to point it at an Arboreal Grazer to get a couple points of damage in.
This is from the third game of a match I recently lost to Tommy Ashton. I was so worried about getting blown out by Turn to Slag, which got me good in game two, that I was overcautious. Instead of attacking for three and using Feat of Resistance if he blocked, then playing a creature if he didn't, I just left up Feat of Resistance. This was bad for multiple reasons.
He now 100% knows that I'm holding a Feat of Resistance because I opted not to play my Igneous Cur which I had gotten earlier with Alpine Huntmaster. Second, losing my Alpine Huntmaster to the Turn to Slag and taking his entire turn wouldn't be that bad if I was continuing to pressure the board. I overvalued not getting two-for-one'd by Turn to Slag while undervaluing pressure.
You can prevent these mistakes through practice. Get the reps in and learn whether you are supposed to run out all your creatures into the Wrath of God if you can't win when they have it anyway because you've slowed down too much. Make plays in practice that you can learn from. Practice isn't about winning, it's about learning.
Extracurricular mistakes are anything that comes from outside of the game. This advice is a bit of a trope since it comes up so often in these types of articles, but the truth is that no guide to proper tournament Magic fundamentals is complete without it. The hardest parts of tournament Magic are eating, sleeping and drinking enough. Magic tournaments are long, but it's easy to forget to eat and drink during them, and your body cannot function without food and water. Bring snacks and a water bottle and turn them into part of your post-game routine. Drinking some water should be as important as de-sideboarding after a match.
Sleep is also super important. My biggest extracurricular fail came in my first Pro Tour, PT Gatecrash. I was 9-5 going into the last two rounds of Standard on day two, and in the middle of my round 15 match my body started to shut down. That's when I realized I hadn't eaten anything that day. I went on to lose that game, and to be honest, I have no idea what happened. I was so hungry that all I could think about was getting out of there and getting food. In game one of round 16, playing for $1,000+, I cast a turn-one Faithless Looting and drew a sideboard card. In my haste to get food, I skipped my post-match routine, and never de-sideboarded. I called a judge on myself, and at the time the penalty for failure to de-sideboard was a game loss. I handily lost that match, and a good showing at my first Pro Tour was ruined by malnourishment.
If you play online, having Facebook up or watching Netflix during your match can distract you and cause you to miss the little things that can be important to winning. Again—focus up, get rid of your distractions, and remember that you are playing a game you enjoy. You should be able to give it your undivided attention, especially if you are playing to win. But remember, eat something, have a water bottle, and get enough sleep.
I hope that this helped in some way. Maybe you didn't even know you were making these mistakes. Magic is complicated game—even the smallest misplays can change the course of your tournament.