Never mind what it was about, it's the simple fact that I am good at communicating, and I was unable to. Maybe I thought I'd understood something, and it turned out that I didn't. Maybe I thought I'd gotten something through, and I hadn't. Maybe I'm trying to get something across, and I just keep failing. Maybe someone's trying to tell something to me, and I just can't understand.
Any way it goes, it's the surest way to send me into a mental state where I'm convinced that I'm not fit to associate with any sane human being -- or insane, for that matter.
[Edit: Those able to explain the subject line get geek points.]