Characters enter a room or corridor, and see what appears to be themselves way down at the other end, just then exiting. If they call to other group, the party will hear it, and, looking back, will see themselves way back where they just were. These are not copies, nor is this an illusion; space has looped around itself in this spot.
Retreating the way they came just brings the characters back to the same spot again. This trap be permanent until deactivated or dispelled, or it may only last for a limited time period, delaying the party in what they were doing (especially useful on adventures where the group has a deadline).
If the trap must be deactivated, there may be a poem or painting on the wall with a clue on how to do it. There may be a hidden switch to turn it off. There might be a puzzle that needs solving. There may be a secret passage out of this corridor or room, bringing the characters back to normal space. Teleporting out, phasing through the walls, or other magical solutions may or may not work, at the GM's discretion.