Using 12.04 LTS - few days ago, I left my machine on while I went out. On my return, the launcher was showing (it is set to be hidden) and there were small numbers 1-9 on the icons and letters s on the bottom-most icon and t on the rubbish bin. System still worked fine, although the launcher remained visible, and would not hide. After about 20 mins work, they suddenly vanished and the launcher hid itself.. is this a normal action ? It hasn't happened since..
It's perfectly normal - those are the numbers you press with the Super button to open the corresponding app.
But it staying for 20 minutes - I guess it is a bug. But nothing serious, so you shouldn't worry unless it repeats.