For everyone, there are some things that drive them nuts. It often seems, though, that people in mathematics and science have an inordinate amount of neuroses bordering on obsessive-compulsive. As an aside, that is one of the reasons I like the webcomic xkcd so much. In addition to the general dorky humour, he seems to get the comical OCD things that I cannot help but think about. Of course, these neuroses often manifest in fairly different ways. For example, I remember making a joke about being OCD when some friends from engineering science were over at my place, only to be shocked by one of my friends arguing that I couldn't possible be OCD because my counter was so full of randomly discarded objects, something which drove him nuts and he had to restrain himself from trying to tidy (clearly, he had never played cards with me... I neurotically straighten the deck, without even thinking about it. If someone points it out and asks me to stop, I actually find it incredibly difficult to restrain myself).
Anyway, one thing that has always bothered me is poor bathroom layout. It seems like something that should be incredibly easy to do when the bathroom is being installed. After all, bathrooms are rooms that everyone has used before, so it's not like a high tech lab where the technicians installing the equipment might not fully understand what they are used for. Today, for example, I went into a bathroom where there were five sinks but only one soap dispenser and one electric hand-dryer. Who thought that made sense?