I am using mostly TESTING repos. Recently upgraded libevent from v2.0.21-stable-1 to v2.0.21-stable-2. Noticed that X started freezing up a/b once a day while using Firefox browser. Mouse just froze, couldn't do anything w/mouse or keyboard. Had to either power-cycle or SSH into the box to kill X, which was spiking CPU to 100% every few seconds. Nothing showed up in any logs. Downgraded back to 2.0.19-stable-3+deb7u1 (WHEEZY) and no more pbms! (I could not downgrade back to v2.0.21-stable-1, since it's no longer in TESTING.