There are mistakes I have made looking back on the games, campaigns, parties I have been part of. Once they happen they are hard to contain, but I have learned from them and try not to have them happen again.

1. Rules as Written (RAW)
   • Following the rules too religiously can take your game down a path where the focus moves from role-playing fun to rule-following rigor. There are so many rules in all of the books it is impossible to have them all memorized. As a DM I’ve learned it is important to be confident to inform your players what rules will apply and which ones will not apply. This is an excellent approach as long as you are consistent in the rules that apply. This discussion should also be part of session 0 so you don’t surprise your players in which rules you are following or not following.
2. Deck of Many Things
   • There are many items that can effectively break the campaign or the party. The Deck of Many Things is probably one of the best known ones. I’ve learned to be very careful in regards to what items the party finds and how long they can exist. Some magic items have the ability to turn a compelling quest into a walk in the park. As a DM I have learned that the magic items need to fit the campaign and the players. There are some magic items which are original and inventive, but do they fit the players and the campaign?
3. Party fitting the Campaign
   • The options for player characters is endless with all the races, classes, and sub-classes now in 5E. But do they fit the campaign and setting? I have run multiple campaigns where the player characters didn’t fit the setting and effectively broke the game with their powers and feats. The choices also limited me as a DM as their backstories didn’t fit the setting. This made side-quests and plot hooks seem awkward and forced. Do not be shy to limit the source books and the race and class choices for players. They may be disappointed at the start but it will make the campaign the richer for it.
4. Making the story the hero
   • Probably the biggest mistake I made was making my story the hero. In my first home brew I viewed myself as the next J.R.R. Tolkien. The Dungeon Master and his or her story is not the adversary of the players but rather should be a fan of the players. The goal of the Dungeon Master is to create challenges and puzzles for the players to overcome, succeed, and grow. Early on I considered my story king and I viewed DMing like writing a story that I would explain to the players and they would love my story. But what makes Dungeon and Dragons special is the autonomy the players have to tell the story within the plot the Dungeon Master has sketched. The Dungeon Master creates a rough outline and the players paint it, shade it, change it, and bring it to life. The beauty of Dungeons and Dragon is that I sketched a castle and that the players painted a temple.

There are probably many more mistakes to be made and lessons to be learned as I am always learning but these are the ones that spring to mind. Another great source of guidance is Sly Flourish and his books and articles.

Sly has a great article on the topic you can find here : https://slyflourish.com/dm_mistakes.html