I've seen a lot of test failures recently that are one-off crashes on MacOS (although some of them occur more than once, e.g. bug 1568385).

These seem to be the usual GC crash signatures we see in the wild, although they must be happening much more frequently to show up in testing.

I'd like to understand why this is happening.

I've made this bug depend on all the examples I could find in the last six months.

Breakdown of which machines these failures are occurring on, where this information is still available:

t-mojave-r7-071 x x x x x x x
t-mojave-r7-079 x
t-mojave-r7-097 x
t-mojave-r7-122 x x x
t-mojave-r7-412 x
t-mojave-r7-458 x x x x x x x