This actually happened twice to me today - a demon runs into a teleporter, which transports him nearby. The demons runs into the teleporter again and so on, until it's interrupted. The first one was looped in a teleporter that had it's destination on an adjacent tile. The second one (a couple of games later) was a couple of tiles away.
I had int 2 in both cases and a tracking map in the second one as well, so I could see that it's an infinite process. See the pictures - both demons are just after teleporting (Hehe... "The demon suddenly disappears! Suddenly a demon appears from nowhere! The demon suddenly disappears! Suddenly a demon appears from nowhere!"). The demon in the first picture was looping long before I entered the room.
BTW, I tried to telefrag the first demon (which ought to be very easy, since it couldn't move from the teleporter destination, but I found out that it's not possible: I got teleported to what looked like a random place instead =(
Also, what does "Suddenly you feel weird!" mean? Is that displayed after every teleport?