Comment 3 for bug 880774

I can't reproduce what you are describing with Epoptes on Ubuntu 10.04 (Gnome), even if I execute tuxpaint --fullscreen.
Could you describe the exact steps you followed?

The truth is that when locking the screen we only block/grab the keyboard and leave the mouse.
But we should grab it too, as I just found out that you can indeed "bypass" (in another way than the one you describe) lock-screen:

If you keep the mouse click pressed - *before* the admin locks your screen - on the title bar of a window e.g. terminal, then the lock-screen app doesn't grab the keyboard and the focus remains on the previous window. So if the window was a terminal, he could execute whatever command.
I think this would be solved, though, if we grab the mouse too, when locking the screen.