I have this problem "sometimes" with 12.10. I noticed, that shortcuts for window moving and resizing are bad as well when this bug is "active" (I have mapped initiate window move a resize window to Super+mouse button, it sometimes switches to Alt+mouse button). When I tried to change "Initiate window move" to Super+LMB in CCSM, the problem with Alt+RMB disappeared as well.
I have this problem "sometimes" with 12.10. I noticed, that shortcuts for window moving and resizing are bad as well when this bug is "active" (I have mapped initiate window move a resize window to Super+mouse button, it sometimes switches to Alt+mouse button). When I tried to change "Initiate window move" to Super+LMB in CCSM, the problem with Alt+RMB disappeared as well.