One of the machines in the break room decided it would start rejecting perfectly normal coins.
It only rejected AtB quarters. It had no problem with the Statehood quarters, with the old eagleback quarters, or with any other coin.
The only difference I can think of is that being newer coins, they have on average higher lustre, and if part of the coin recognition mechanism is optical, that might have something to do with it. But as far as I know, the mechanism is based on size and weight, not reflectivity. Even if there were an electric eye for detecting washers, an AtB quarter should pass that test since it's definitely not holed in the middle.
Weird. A real coinundrum. Any ideas?