Since pjc wanted a example that isn't explicitly a bug (a 'benign' race condition) I'll provide one that can be a major pita, two concurrent tasks are logging their event stream. Logs may not hit in exactly the order that you expect in your mind, forcing you to stop and think each time you're going over the logs. Definitely a race condition. Definitely expected behaviour. Not sure you can call that a bug.

More than once in my life I have thought that this was a bug, tried to correct it, and introduced another bug, followed by teeth gnashing, hair pulling, swearing, and then a revert.