Steps to reproduce:
1. lock the screen
2. change the screen resolution
3. press Escape at the unlock dialog


Stack trace:
No stack trace yet, I'll try to get one tomorrow. I'll paste the assertions that failed below, that might already provide the needed information.

** (gnome-screensaver:25854): CRITICAL **: gs_window_is_obscured: assertion `GS_IS_WINDOW (window)' failed

(gnome-screensaver:25854): Gtk-CRITICAL **: gtk_widget_get_screen: assertion `GTK_IS_WIDGET (widget)' failed

(gnome-screensaver:25854): Gdk-CRITICAL **: gdk_screen_make_display_name: assertion `GDK_IS_SCREEN (screen)' failed

(gnome-screensaver:25854): Gdk-WARNING **: /build/buildd/gtk+2.0-2.14.4/gdk/x11/gdkdrawable-x11.c:878 drawable is not a pixmap or window

(gnome-screensaver:25854): Gtk-CRITICAL **: gtk_widget_get_screen: assertion `GTK_IS_WIDGET (widget)' failed

(gnome-screensaver:25854): Gdk-CRITICAL **: gdk_spawn_on_screen_with_pipes: assertion `GDK_IS_SCREEN (screen)' failed


Other information:
The steps to reproduce are very condensed and contain some assumptions and leave out a lot of information that might be helpful. I'll just paste you the detailed description from https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/314671 with slight modifications:

I'm using Ubuntu Intrepid for AMD64 on an Intel Core 2 Duo, an ATI card with the closed-source fglrx driver and the proprietary ATI Big Desktop for my two-monitor setup. I'm not entirely sure what exactly triggers this bug so I'm hoping for others with a similar but slightly different setup to track this down. What I do to reproduce is lock the screen, wait until it goes blank (or close the lid), wait some more seconds for my external monitor to go to standby and wake the screen up again to make the password prompt appear. Then, instead of entering my password, I simply press Escape and voila, screen unlocked (what should happen is that the screensaver itself reappears). What tells me that gnome-screensaver crashed instead of just letting me through is that I have to enter "gnome-screensaver" in the console to be able to lock the screen again.

A wild guess what might have gone wrong: When the screen is turned off, the external monitor goes to sleep, the screen size is halfed from 3840x1200 to 1920x1200 (which is an annoying thing to happen anyway but that should be another bug report) and gnome-screensaver somehow can't cope with that change in screen size. Consequently another way to trigger the bug is switching the monitor off and back on (only switching it off somehow doesn't seem to change the screen size and it surely doesn't trigger the bug).

Also, sometimes the screensaver is displayed in a window after the screen has been miraculously unlocked. One called "slideshow" showing Cosmos in my case.