(found in SVN 1061)
When a domain's quota is set to "0", meaning "unlimited quota" according to the web interface, all edits to a user's quota are rejected as too high.
The problem is in the function "check_quota", which checks
if ($quota > $limit['maxquota'])
$rval = false;
AFTER it checks
if ($limit['maxquota'] == 0)
$rval = true;
and therefore declines any quota for being greater than the "maximum" of 0. Reversing these two checks fixes the problem and causes a quota of 0 to return a successful quota_check as expected.