In Lords of Madness (3.5), there's a beholder magic item called a Sphere Mirror.
"This large pane of glass measure 10 feet tall by 5 feet wide. The item's name, which seems at odds with it's rectangular shape, actually refers to the spherical shape of a beholder's body.
The glass surface of the mirror does not reflect light, and viewed under normal circumstances, it seems like little more than an empty metal frame. The magic of the glass perfectly reflects the image of any beholder that looks into it, up to a range of 60 feet. The image is so perfect that it is very likely to be mistaken for a live beholder; a DC 30 Spot check reveals the truth but offers no addition protection from the beholder's eye rays.
A beholder can reflect and amplify any of its eye rays off of the surface of the mirror, effectively firing the ray from the location of the mirror rather than from its eye. The beholder needs line of sight to the mirror, but not line of effect. If it fires a ray at an object it cannot see from its actual location or the mirror's location, the target gains the benefit of total concealment. It is possible for a beholder to redirect its eye ray attack around corners if multiple sphere mirrors are used, as long as no two mirrors are more than 60 feet apart."
This goes along with the fun house mirror concept Olgar mentioned. Imagine a twisty complex with your "eye ball" at one end with a dozen sphere mirrors. Each time the group "kills" the "beholder," the walk another 60 or so feet only to encounter another "beholder." And since the mirrors are magic items, you can also enchant then with other magic, like enlarge image or alter image, so that each "beholder" they encounter looks different.