I continued to drive mine for about 6 months with it happening, from what I can remember what Doug told me was the nut backs off more and more over time and starts to grind against a plastic oil return which starts to leave chunks floating through your gearbox causing machine gun rattling in all gears (did not sound good!).

If the popping out has only just started occurring and you get it fixed ASAP your bill may be less... food for thought I guess.