[AlotOfReading]

That's just the sort of stuff I see, but long tail issues that take weeks or months to diagnose can happen anywhere in the stack. Sometimes it's a hardware issue, sometimes it's a compiler bug, sometimes a VM bug, a missing browser feature, etc. Any of that can crop up even with "trivial" changes. I'd argue that it's not useful to include that in your estimates, but they're nevertheless possible and need to be incorporated in scheduling.