I consider myself fairly computer savey, I've built a half dozen + computers without problems, but sometimes my common sense fails me.
So there I was, drinking, (not really, but I think all good stories should start out that way)putting together a computer for my friends brother. He had purchased all the parts, and asked me to assemble it. So I go over and put in the processor and motherboard fine, and I go to put in the power supply and the hole for the cable on the back is in the wrong place, so I can't access the plug. I'm trying to figure out what kind of case he ordered, in the mean-time he calls his father who brings over some tools. We get halfway through cutting new holes and go to fit the power supply. The 50 year old man starts to put the power supply in upsidedown (or at least opposite how I initialy installed it), I didn't stop him and to my suprise it fits perfectly. I just put it in upsidedown to start with.