Due to a change in behavior for OS X 10.8 (seen with 10.8.3), the code added to posixmodule.c in Issue7900 to handle an unlimited number of groups no longer works.  The code depends on the documented behavior of getgroups(2) failing with EINVAL when the grouplist array in the call is too small to hold all groups.  This works correctly for 10.6 and 10.7.  Currently in 10.8, such a call succeeds and truncates to the first grouplist-size groups.  The getgroups function could probably be modified to always call getgroups(0) first to get the real length.  But it seems to be a clear regression in 10.8 and breaks existing code.  I've opened a bug report with Apple about it.  I'll plan to keep this incident open until I hear something back from them.