Be afraid to write things down
This one's divisive because many servers feel there are pluses to memorization (more eye contact, personalized service, writing isn't cool), but the reality is that, unless you're perfect, this leads to mistakes. So if you have a savant-like memory, congratulations! Throw your pad in the trash. And begin practicing for Jeopardy!
Not repeating back the order
At least once.
Say "are you still working on that?"
Eating is not work. The customer isn't clocking out at the end of the meal.
Gossip on the floor
Your restaurant should appear like a heavenly, bitch-free zone where anyone would be happy to spend their days.
Call in sick and then Instagram yourself at a swimming pool
Everyone follows you, and now everyone hates you.