I don't know if it appears in all of the broken/unused shredders in the game, but I noticed in the northern part of The Barrens by the Horde entrance to Warsong Gulch there are two such shredders. If you look closely at the control panel inside of the shredders, right in the middle between a couple levers and joysticks there is a picture.

It appears to be a face, and it's actually a picture from the CD cover of the game Brood War (the expansion to another famous Blizzard game called Starcraft). The face is that of infested Kerrigan. I guess Blizzard wanted to include a little Starcraft into the World of Warcraft.