Steps to reproduce:
1) Open NeoLemmix in windowed mode and start a level.
2) Drag another window over the skill panel.
3) Minimize this other window.
Then the text line on top and parts of the skill panel are not redrawn properly. Everything still works fine, as long as one only drags the other window over the skill panel, though.
This bug is pretty old. I compiled from a commit on March 9th and the bug was present there, too.