[LorenPechtel]

What's especially important is non-obvious state.

I was trying to use a function. You had to create an instance of it to use it, but I was using one method that by appearances should have been static. You feed it an input, it gives you an output, there's nothing to remember. I was calling it from multiple threads--occasional corruption. There must have been something being stored into the object but I have no idea what or why.