Not sure what you're trying to explain, I didn't see a mention that the insert method is thread-safe (that would have been sufficient, but also required). It's your code after all, so I guess when you get burned, you're doing it in a very assumed way. Bugs of these type can show up after executing many trillions of cycles, or instantly, depending on the input. Having identical outputs after thousands of runs means nothing, since the core issue is unaddressed.