We all make mistakes and no one is perfect.
Those are two too much used phrases but they're sadly true.
Of course they're true also in a rpg, since it's one of the mirrors of the world.
I've been playing and DMing for almost 25 years and I've seen a lot of mistakes.
I've made many mistakes, and I'm still making them.
Here's a list of the most common:
Desire to Win. It's not a fight between you and your players. Your satisfaction isn't about crushing characters or putting them in a trap without hope to escape. You "win" when your players, after the session, tell you they've fun and want to come back at your table.
Wrong Rewards. You spent hours crawling a dungeon, escaping from a lot of deadly traps, killing two beholders, banishing a lich and defeating an entire hobgoblin tribe. You get 200 xp and a couple of magical boots, both left feet and gnome size. What do you feel, as a players? Behind that behavior there's the "characters are going to become too powerful and I'll lose control of my game" syndrome. Don't be afraid of it! You're the DM, and you're a good one. You can surely make high standard challenges for them. Remember: players need to see their characters improve, both levels and powers.
Deadly Encounters. They come from Desire to Win. You force characters only in very hard fights, with a lot of troubles related to: many deaths, run out of all resources at every fight, need to "recharge" by rest after any encounter. By doing that you only frustrate your players. Make some changes to your average encounter difficulty: better a couple (or three) of medium difficulty fights than a deadly one.
Impossible Challenges. It's another syndrome, similar to that above, known as "Characters are too powerful!". PCs have grown in levels and now they've got more power than before. You reply, with the purpose of nullifying their stronger powers, by suddenly increasing any monsters' AC, so they can be hit only by a natural 20, or by having check or saving throws with very high difficulties. By doing this what is the purpose of level advancement? Take it easy, the game is well balanced, expecially D&D 5th edition, and can have believable encounters and challenges even at highest levels.
Long Speeches. Did you write pages over pages about your world's stories and you want to tell them to others? You should be writing, not DMing. All speeches or descriptions longer than 5 minutes lower your players' attention. They may risk to lose the important information at 19' 28" of your speech. Flow informations little by little, giving to your players the time to think about them.
Desire of Supremacy. You are the DM and you're the uncontested boss. You know and I know. But remember that the story is about characters and that players move them. The game is centered on them, not on you and your creations. Forcing a party to only watch to all main events of the campaign without any chances to act or to make choices does mean you're going to kill the role-playing. Create your events and situations related to them and let PCs free will to act and reformulate those events according to players' moves.
Railroad. You've just prepared an adventure, a campaign or a session thinking your players would have taken some decisions and then they take some completely different. You're under pressure and you react by blocking their moves, even if your behavior seems illogical and it's clear you are opposing the party on purpose. Know that players make decisions you didn't think about and you've got only one solution: improvise and quickly change the session according to the new situation.
None preparations. It's a common reaction to avoid Railroad or troubles about plot twists which delete hours of work. It's wrong as much as Railroad. To not arrange anything means to not be ready at any players' action and soon or later you'll be caught in fault. Focus only on fundamental things but prepare them very good.
Hesitation. You must make a call or decision but you don't know which one. To think about too much slows the flow of the game and lessen its positive tension. Make your call as soon as possible (max 1-2 minutes) trying to favoring, if doubtful, game pacing and players' satisfaction. You can warn the player you'll take another look at the case after the game, when you'll have more time, and that you can change your call for the next times. If you're in trouble between two choices you're in a hurry, flip a coin, but decide quickly.
Table Control. One is thinking about his own stuff, a couple speak about private affairs, another one is complaining about everything and she has a lot of rule-lawyer arguments. It's not a game table, it's a bazaar and it kills the game. As DM, I'm really tolerant (players can check their phones, can make little jokes..), after all we're around a table to relax ourselves from real life, but when things become important players are all attentive. Players can give their opinion about a rule and sometimes they made me change my call, but when I make it definitively, the game goes on without other discussions.
Think about those mistakes and try to make them less and less in the future.
It's a good exercise to improve your way of DMing.