1) believing you can dramatically change the performance of an employee -- it's very rare to save someone and less experienced managers always believe they can.
1.5) corollary to the above: not realizing the team is aware and waiting for you to fix the problem and won't thank you for taking longer to do what's necessary.
2) believing that people don't know what you're thinking -- people see you coming a mile off.
3) thinking you can wait to fix a compensation problem until the next comp review -- everyone waits too long on these.
4) believing HR when they tell you that you can't do something that's right for your team -- what they're really saying is that you have to go up the ladder until you find someone who can force them to make an exception.
5) not properly prioritizing the personal/social stuff -- at least this is my personal failing, and why ultimately management has not stuck for me.
6) believing your technical opinion matters -- I've seen way too many VP's making technical decisions that they are too far from the work to make, trust your team!
It'd be fun to see a list of these from the non-management point of view. I'd start off with the inverse of #6 above:
1) believing your technical opinion matters -- the business is what ultimately matters.