The git commit says "focus vs modify race" so it sounds like a race condition. I don't understand the "notion" either - to me it reads that the probability is

The git commit says "focus vs modify race" so it sounds like a race condition. I don't understand the "notion" either - to me it reads that the probability is so low it happens for a pair of operations once every 500 million operations or so. If it is a race condition you might have the issue after doing just two operations.