If you are running a program with a custom layout, for example Russian on a normally English system, that can interfere with you entering screensaver password.

Step to recreate:
1. Set a password using an English keyboard layout.
2. Add Keyboard Indicator to panel.
3. Open Office Writer.
4. Change the keyboard layout to Russian for Open Office using the Keyboard Indicator.
5. Go have a cup of coffee and think about how your microwave can be used in the quest for world domination. This should take long enough for your screensaver to be activated.
6. Go back and enter your password to unlock the computer.

Result: You are unable to access your computer. You have to kill or restart the X server/gdm to gain control again.

Wanted result: That the screensaver does not take a non-default keyboard layout

HTH