What are some common mistakes to avoid when writing copy for UX?

IMHO, the most dangerous is to suppose, that users care about your company. 'Help us to be better' and stuff like that.

A big one, in my opinion, is when your copy lacks 'emotion'. As a writer, I cannot afford to give instructions like a programmed robot and expect my audience to connect with my product. I need to move my audience from Point A to B without them feeling like they are completing a chore and a copy with 'emotion' does just that.