Metropol

From http://en.wikipedia.org/wiki/Thirteenth_floor

"It should be noted that to place a floor between those accessible from an elevator, it is necessary to either take longer to travel between the neighboring floors, or accelerate, both of which would be noticed by the riders. Making use of an artificial wormhole to jump past the thirteenth floor might be considered as a solution for this, but this would entail other practical problems."

I do quite like this one. Mainly because I spent two weeks trying - and succeeding - in understanding it!

A clever, clever man wrote:The action of a passing Gravitational Wave is to change the curvature of spacetime and hence the apparent separation of two or more objects.