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.