A quick example:
"Paradox of Choice" - Giving a user more control via options, configuration, and settings is counter-intuitive from a UX standpoint. We expect users will relish their total freedom of choice when, in reality, it causes anxiety and decreases satisfaction with the outcome of the task.
I'm talking about psychological mind-bombs like this. I frequently cite Barry Schwartz's idea when working with developers who insist that a simple option or two (or three) is all that's needed to solve a problem.
It's easy to balk at this idea because we personally value the freedom of choice. Unfortunately, few people assess its negative side-effects which can directly impact UX.
What are some of your other favorite ideas and principles like this?
Update: found a relevant resource here http://uxmyths.com/