Blank icons for some programs on dock

A few days ago, one of the programs I run frequently (World of Warcraft) began appearing as a blank icon on the dock. By "blank icon" I mean there is nothing there but an empty space, through which you can see the background image. There is the little lit circle underneath it, of course, to indicate a running program. I do not keep in icon for this game on the dock.

Today, it started happening to some other programs. As with World of Warcraft, this is only happening (so far) to programs I do not keep on the dock when they're not running. It's happening to Firefox, and just recently, it happened for System Preferences. However, it does not happen for all programs that aren't kept on the dock. I tried running two other programs, one that came with OS X (Chess) and one that did not (some game like Windows' Minesweeper), and it didn't happen for either of them.

What should I do to fix this?