I don't work at Google (any more) but I have seen bugs in large-scale production that served one user's data to another user, and both times it had the same cau

I don't work at Google (any more) but I have seen bugs in large-scale production that served one user's data to another user, and both times it had the same cause: a developer stored user-specific data into a process-global singleton because the consequence of some java decorator was non-obvious. When the next user request came along they were served the previous user's information.