Did you do the wiring yourself? If so, anyone can goof, that's why there are testers. If you fix it yourself, it could be wrong either in the circuit breaker box or in the wall. It is easiest to check and fix in the box. Naturally, it is most likely to be in the wall, where it is a pain to fix--the heavy-gauge wires are hard to move around in a small space. If an electrician did the job, he or she should fix it for free. I've installed the hot and neutral backwards in my house, but so have professional electricians. I had a bathroom redone where this happened, and also where the plumber installed hot and cold backwards. Easy to fix under the sink, but the toilet was being flushed with hot water! Fortunately, I was able to point out a spot in the room on the floor below where the pipes could be accessed so they didn't have to tear out tile, they just had to do a bit of fancy crossing of copper pipes.