[TYMorningCoffee]

Oh no I didn't know this can happen with HashMaps too! Something to do with the linked list they use for collisions?

[scottlamb]

There's a dead comment replying to yours with this link: https://mailinator.blogspot.com/2009/06/beautiful-race-condi...

...which I don't think I've ever seen before, but nicely explains the problem I saw, and coincidentally is from the same year!