We woke up this morning with an accepted reservation not approved by us. How can that happen?

[Kym0]

You probably have the Instant Book option turned on. I believe the dafult setting on that is to allow "anyone in the Airbnb community" to reserve without your acceptance. Go into edit mode under the Booking heading. The 3rd or so item is "Instant Book". If you see a yellow thunderbolt, that's the problem.