I bought a new tandem and immediately experienced the joys of many flats. I knew that tandems flatted more often, but twice a ride was getting a bit much. I thought it was my technique: Pinch flats, every one of them, right after a bump. Then I looked at the tubes. The holes were on the INSIDE of the tubes. I looked at the rim and found that the rim tape had missed the tiniest part of two spoke nipple recesses, smaller than this |).
I had already checked the rims for rough spots three or four times. If it wasn't for a rainy morning with no possibility of a ride, I would have never found it.