I just recently posted about rarely seeing multithreading problems, and now I get called into work in the evening because there are reports of users seeing data for other users, and this started just after a new build went out to production. Since it was in code that I had never looked at before, I had to grep around to see what was calling what, since the lowest level code that I was initially pointed to looked fine. In a class three calls up, there was a suspicious looking static declaration of a User object. Looking at the history in source control, it had been changed to static for no apparent reason, not even a stupid reason, such as using it in a static method.