Here are a few...
1. Thinking cover songs need to be played just like the original (especially if you think you need the specific effects to play a song). Special Note: Keyboard players activating a clavinet patch seemingly for the first time just to play Superstition badly: I'm looking at you!
2. Not realizing you are performing the entire time you're at the gig (includes speaking to the crowd between songs, setting up/taking down, behavior onstage/offstage, before/after the set, etc)
3. Discussing things onstage with a hot mic.(extra credit though to one of my drummers for lamenting the quality of a backline kit with expressive profanity - that was pure gold!)
4. Tablature
5. Understanding stage volume and EQ vs PA (guilty!)
6. EQ'ing sound for one's own instrument (ie best bedroom sound) instead of band as a whole, and how the audience hears it.
7. Thinking people actually care about your TOAN!
8. Sticking to a setlist instead of reading the crowd
9. Disparaging the sound engineers publicly (see #2)
10. Sound engineers EQing everything for max volume without feedback, or EDM-type bass response regardless of vibe of the band.