Never overestimate the difficulty of concurrent code

Whenever anything goes wrong in concurrent code, people assume it's because of the concurrency. Especially if it's an apparently-random error, e.g., memory corruption.

But that doesn't actually mean the concurrency is the cause.

Turns out that I read the MSDN page wrong- it was false for "insertion did not occur", not false for "value did not already exist". Whoopsie.