Was actually about to post about this just now. I actually just discovered this on a bike I bought used recently. Took the wheel out and determined that the replaceable right side dropout had an enlarged hole (11.2mm compared to the 9.8mm on the left side). No idea if it was a defect, wear, or messing around on the previous owner's part, but I have a new dropout on the way. So check that if you haven't already.