For example, thread 1 produces a value, and writes it into a member variable of a class. Thread 2 uses that value, but does not synchronize with thread 1 to make sure that it's been produced. But that's fine, because thread 1 finishes before thread 2 uses the value. That is, thread 1 almost always finishes first. But if it doesn't (if it loses the race), then chaos happens - chaos that is very hard to reproduce or debug.
