I've never seen that before. Weird.
I saw this quite often in V6 but only a couple of times in V7. Nobody else in my office had the issue. Nor did they see it when I sent them a packaged workbook where I was seeing the issue. I reported it once to Tableau quite a while ago but nobody recognized the issue. I chalked it up to some weirdness on my PC. The problem would clear up if I closed my Tableau session and restarted. Sorry I have no solution for you but at least you know you aren't alone.
I could have sworn I had tried closing and re-opening - but I just did that again and it has now corrected itself, so thanks for that Tim.