I bought my wife a new bike and she said It is "slipping". I took it for a ride and sure enough, it feels like it is slipping just a tad on every rotation.
Its not an in-between gear or tooth slip. It feels like the pedal crank slips a 10th of a turn on every rotation.
Any ideas where to start looking? It is a mountain bike with 18 gears, and has only been ridden for less than a 1/2 mile (800 metres).