The warnings that M. Siebennmann hit (since this has come up again this year) were going to standard error. If you write to standard error in a cron job, mail

The warnings that M. Siebennmann hit (since this has come up again this year) were going to standard error. If you write to standard error in a cron job, mail gets sent to root. All of the cron jobs using fgrep and egrep were suddenly mailing root, every single run, even when nothing went wrong.