Have you ever read Arthur Porges' short story "The Mirror"? Basically the plot is as follows: a single-parent family buys a house dirt-cheap after a man goes mad and massacres his wife and all his children. Inside it they find a strange old mirror covered with paint - but the father admires it greatly, so he removes the paint and sets it up in the living room. Then, to josh his kids he makes up a story about a little creature called Gnolfo who lives inside the mirror but cannot get out. The children, being children, both believe and disbelieve his story.
One night the father goes out on business, leaving the older children to take care of the younger ones. Naturally they get into mischief, and one of the others pretends that he can actually see Gnolfo in the mirror - the younger ones get nervous and say this is a lie, whereupon one of the others says that he actually has seen Gnolfo, and that the reason they can't see him is that he has gone behind a walled cavity out of eyeline in the mirror-world room. To remove all error, he comes up with a wonderful solution: the children will carry in the mirror from their father's' bedroom and set it up so they can acquire an angled view of Gnolfo behind the barrier in the other room.
The do this. At this point some kind of gigantic centipide-like organism with huge talons appears in the mirror, moves around a bit, then senses them, comes towards the foreground of the first mirror, passes through it and rends them all to shreds. The father returns home and discovers their bodies, goes mad and is indicted for their murder.
So no, I don't think I'll be doing that.
The effect is used in one of the greatest shots in any movie I've seen, in Citizen Kane, when Orson Welles is walking down the hallway and we see him projected in a mirror about a dozen times. It's supposed to show Kane reflected in every stage of his life or something symbolical like that. Immediately after seeing it I tried the effect with a lit candle. Was pretty cool.
-Kev