On a fresh install of WordPress, the admin user begins with user_level == 0
This seems rather serious to me.
It's easy to check:
global $current_user; get_currentuserinfo(); echo "level:".$user_level."<br />"; exit;
This was not a problem in 2.3.3; it is a problem from 2.5 onward at least through the current trunk.
All of my test systems have user level 10, so it somehow gets "fixed" after running a while. However, in the first week of my plugin's use, I have had several reports that my plugin is broken, and I traced it down to this issue.