What is the worst mistake you have to watch for?

[s0cks]

Tension. Always tension. Be it a tense finger, hunched shoulder, bent back, etc... I'm working on this exclusively now because I can see how powerful a tool it is to be able to play super relaxed with the least possible effort. Makes a world of difference with my connection to the guitar, and my playing ability. I'm super excited to rid myself of old, bad habits, because I can already see this making a cataclysmic improvement in my playing.