Why has my OS X dock icon disappeared?

So after finding that one of my dock icons appeared pixelated, I recently experienced some of my dock icons become invisible.

I’m not sure what caused it although I think I was running low on disk space at the time so it may have been memory related. All I did to fix it was re-drag the application into the dock and the icon reappeared. There’s also a discussion in the Apple support forums about disappearing icons.

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Ben Huson