I have those fairly frequently. Usually I'm in high school, trying to take classes that I skipped over, because they suddenly decided I had to take them or something like that, even though way beyond college at this point. Usually, somewhere along the way, I suddenly have this moment of realization like "Wait a minute -- This is ridiculous! I got my BA back in '93 and they've got me back in High school now? -- What on earth can happen if I fail?" And then I usually wake up, still rather anxious and flustered, unfortunately.