I once had a co-worker who did that. The way that the work spaces were set up there was kind of annoying to begin with since the entire R&D department was basically one giant table and all the programmers were seated around it. So we didn't have our own cubicles or anything. Then they hired this weird guy who ended up seated next to me and he talked to himself all fucking day. He had some sort of mental condition (never did figure out what but there clearly was something wrong with him). It was irritating as hell because it's quite hard to concentrate when you're listening to someone else's thoughts and also it is just unsettling to listen to someone having a conversation with themselves (which he often did, he'd ask himself a question and then answer it).
