Comment 17 for bug 390989

This issue is related to the fact that the screensaver must grab the keyboard and mouse, but can do so if other applications already have a grab on them.

There is no current solution to this problem with the way X is designed.

See:
https://wiki.ubuntu.com/DebuggingScreenLocking#Screen_is_shown.2C_and_then_screensaver_activates_again