Just notice this method throwing while testing a13. The problem is in this line (github permalink). It seems it is possible to create identical HideInCallstackGenericMethodKey structs, so that line will throw when the key is already there. The immediate fix is to use TryAdd(method) and bail if returns false. I will see if I can submit a proper bug report, but I'm a bit pressed for time right now, so I decided to at least get this threat going in the meantime. Thanks for reading!