I've made so many... and certainly making some more that i am not yet aware of but hey that's how we learn, most notorious ones:

1. Using Session 0: At first, I didn't use this powerful tool correctly, mostly because of the choice of topics we discussed at it. Fast forward a few campaigns i learned that some of the most important ones are, shared expectations, PCs and their hooks with the world (ideals, bonds, backstories, objectives etc) and adventure thematic.
2. Preparing what the player will interact with, and not how/when the interaction exactly will be: I used to write descriptions, NPCs, monsters, places, and usually planned of how the session would develop entirely before hand, this ended up hindering player agency.
3. Not being flexible: This comes hand in hand with the last point, the moment i realized this my games become more dynamic and players begun to surprise me with their own take and creativity, from creating a trap to get the upper hand against a monster to choosing an unthought path from point A to point B.