There was a generation of date processing in Java when the class wasn't reentrant. The API was functionally flavored though, so people would instantiate one instance configured how they liked and reuse it across all calls. You would only see this problem under heavy load, such as in production, and it basically took encountering someone for whom it had already happened to spot what was going on. I think I found out from the Java issue tracker, thanks to some other dev filing an accurate bug report, but then I was that person for others at a full handful of other jobs afterward. Everyone was surprised, as one would be. They eventually fixed it but that was busted for a long long time.