I noticed a case where mod_xrandr.rearrange_workspaces() seemed to hang.
Exact symptoms: I expected a workspace to be moved from my second screen, which I disconnected, to my first screen. The previous workspace did not disappear, but it seemed like the to-be-moved workspace was active: the mouse pointer behaved as if it was over the to-be-moved one, and i could even open an xterm and execute a command (though all that was invisible).
Switching to console I noticed firefox was taking a lot of CPU. Killing firefox caused the workspace rearrangement to be completed and my machine was normally usable again.