[bmm6o]

I've seen this in production C#. Same symptoms, process dump showed a corrupt dictionary. We thought we were using ConcurrentDictionary everywhere but this one came in from a library. We were using .net framework, IIRC .net core has code to detect concurrent modification. I don't know how they implement it but it could be as simple as a version counter.

It's odd he gets so hung up on npe being a critical ingredient when that doesn't appear to be present in the original manifestation. There's no reason to think you can't have a bug like this in C just because it doesn't support exceptions.

To me it's all about class invariants. In general, they don't hold while a mutator is executing and will only be restored at the end. Executing another mutator before the invariants are reestablished is how you corrupt your data structure. If you're not in a valid state when you begin you probably won't be in a valid state when you finish.

[TYMorningCoffee]

It came down to poor logic. I got hung up on it because I got unlucky and couldn't reproduce it with uncaught NPE so I incorrectly concluded that uncaught NPE was a necessary condition.