The quota for this user has not changed. Only turning quotas off and back on does the trick.
This is bug 5660. Patches have been made for the past several versions which do fix the problem, and I imagine the patch will be made available for 5.0.2 as well. Be sure to let Customer Support know if you're interested. The bug has been fixed in the release that will follow 5.1.
Paul Eastham Network Appliance Engineering