Seems like this has to be a known bug (it was present in beta 11, and I didn't report it then since I just assumed it was in the bunch of other HUD things being fixed), but I haven't seen it mentioned in the threads:
If you have a table minimized while the HUD is running, and then bring the table back up, that table is littered with un-erased old showdown cards in the middle and old pocket cards on the player's names. It doesn't go away automatically - perhaps it eventually goes away the next time you see a showdown, but that can easily be several minutes.
It seems like this would be an easy fix -- wipe the showdown area clean each time a table is restored or maximized -- I hope so. As it is, I have to toggle the HUD off and back on each time I sit down at a table that I've had minimized while I was on the waiting list.