Sound like a critical race condition or bad memory access (this latter only in languages with pointers).

Since it's HTTP(S) and judging by the average developer experience in the domain of multi-threading I've seen even for people doing stuff that naturally tends to involve multiple threads (such as networked access by multiple simultaneous clients), my bet is the former.

PS: Yeah, I know it's a joke, but I made the serious point anyways because it might be useful for somebody.