Trackpad gestures stop working
In 10.8, running killall Dock seems to bring back trackpad gestures.

When a reboot didn't fix this, I went to System Preferences → Trackpad → More Gestures and changed


Mission Control to ‘Swipe up with three fingers’, and
App Exposé to ‘Swipe down with three fingers’.


Those gestures now worked with 3 fingers. Then I changed it back to ‘Swipe up/down with four fingers’ on both and they started working again using the four-finger gestures.