Recently I've encountered several instances wherein my Dock crashes (by crashing I mean hovering over it doesn't show and doing Command + Tab doesn't switch applications)
Up till recently I've just been doing a restart but since I got tired of doing it always I tried hijacking it the *nix way:
ps aux | grep Dock
The output for mine was this...
cyx 2118 0.0 0.0 75452 372 s005 R+ 3:12AM 0:00.00 grep Dock
cyx 1195 0.0 0.5 408528 11032 ?? S 5:43PM 0:06.36 /System
/Library/CoreServices/Dock.app/Contents/MacOS/Dock -psn_0_499834
So I issued the usual killing blow:
kill -KILL 2118
It automatically restarted, and things went back to how they were.
Note: This occurs when I frequently do a Command Tab between applications, and I mean frequently, with a ton of open apps too.