We have a bunch of iteration APIs for things like realms and compartments that take a callback function and call it with a pointer to thing being iterated. It would be bad if those callbacks caused GC to happen since this could potentially delete the passed pointer.
There are some mitigations in place: AutoPrepareForTracing is used in some iterators that finishes the current incremental GC (but this is not used everywhere and doesn't prevent new ones from being started). The realm iteration functions pass a handle to a realm (but there's no read barrier to prevent a realm read during an incremental GC from being collected by a subsequent GC slice).
I don't think anything bad happens at the moment but it took me a while to convince myself of this. Therefore I think it would be useful to make things more obviously correct by outlawing GC during these iteration callbacks.