I was doing some testing of zone GCs today to try to track down the cause of the assertion failures in bug 1016738 and I found this problem. This means that, whenever we use GCZoneGroupIter or GCCompartmentGroupIter, we're potentially iterating over zones/compartments that aren't being collected. I'm not sure what the consequences of this are. We rarely do zone GCs, so it's not likely to happen too much. But zone GCs can be triggered by allocating a lot, so it's a potential security issue. Looks like a regression from bug 982561.
Somehow I ended up filing two bugs for this?