gnome-screenshot runs into an endlees loop when capturing a fullscreen
picture. This may be related to a similar bug in reported for GTK 3.8
described here: https://bugzilla.gnome.org/show_bug.cgi?id=695132

Apparently, this happens especially in scenarios when a compositing
window manager like compiz, unity, or gnome-shell, changes opacity
settings during the "fade effect" of gnome-screenshot, so the final
opacity setting of (default) 0.1, that is checked for in
src/cheese-flash.c, is never reached.

A possible solution could be completely removing the "flash" effect
timer loop. Instead of this, in the patch attached, the final opacity
setting has just been increased to 0.05, which keeps the visual effect
working, but seems to be high enough to circumvent the iopacity setting
rounding effect that triggers the inifinite loop.