As a QA analyst, I'm often presented with similar situations. Any time I test something, if I don't get the results I'm expecting, the first thing I do is assume that I did something wrong, and recheck all of inputs to and outputs from my test (unless it's really obvious to me that I didn't do something wrong). Even us testers, whose job it is to be RIGHT, aren't immune to messing up simple things.
It's pretty often, too, that other people will come to me claiming that something doesn't work. The more basic the problem, the more suspicious I am that they didn't screw something up.