With the "snap_inverted" option set (so, with window-snapping on by default), I can occasionally get a window to "bounce" between two possible snap states.
I've only ever seen it happen at the screen edges, and only the top & bottom edges (with gnome-panels on them). For example, I managed to get a gnome-terminal to bounce in the bottom left hand corner. The bottom would stretch out and snap to the bottom gnome panel and then snap back off about once a second, without me touching the window at all.
It also seems to happen most just after a window has been un-maximised.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
Essentially the same as http://bugs.opencompositing.org/show_bug.cgi?id=1047