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

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.

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