This is a really old bug I have with PrBoom.
Look at this screenshot:
http://img695.imageshack.us/img695/7483/doom07.png
and this one:
http://img269.imageshack.us/img269/1152/doom13.png
There seem to be a priority issue in the screenshot action.
I did not see this bug in another game, even in ZDoom.