On RHEL 5.2, if I run 'crontab -e' and use this for my crontab file:

    MAILTO=

    # update rawhide trees daily at 6:00 AM HST
    0 6 * * * /home/released/sync

I get this error when crontab tries to install the new crontab file:

    [root@tenon released]# crontab -e
    crontab: installing new crontab
    "/tmp/crontab.XXXXIhC21M":1: bad minute
    errors in crontab file, can't install.
    Do you want to retry the same edit?

But I use this for the crontab file:

    MAILTO=""

    # update rawhide trees daily at 6:00 AM HST
    0 6 * * * /home/released/sync

crontab is happy.  I know the manpage says to either define MAILTO to something or use MAILTO="", but on other Unix systems (such as Solaris), you can set MAILTO= and it has the same effect.  I wouldn't consider this a huge bug, just a user experience issue.  This is the kind of thing that would likely end up on a Solaris admin's blog under something titled "why I hate RHEL..." because he spent a good portion of his day trying to figure out why crontab was choking.  Well, that's just a theory anyway.