When you write scripts that run as cron jobs, and send email to people, and have the potential to send a *lot* of email to people you really don't want to screw up.
Unfortunately I did screw up when writing one of these. It was a pretty simple 200 lines or so python script that would find any new revisions that had been commited since the last time it ran, and email commit messages to developers.
The idea was simple, a file kept a list of the last seen revisions, I would go through the archive find new revisions, mail them out, and then finally write out the file with the latest files.
Spot the bug, or at least the design error? When our server ran out of disk space, the stage of writing out the the file with the last seen revisions failed, and created an empty file. So next time the script ran it thought all the revisions were new, resulting in thousands of email about revisions committed years ago. I pity our poor sysadmin who not only had to deal with out of disk problems but now also with a mail queue with thousands of messages.
Solution to the problem of course is try and write out the new revision file before sending the email, and write it to a temporary file, instead of blasting the last known good out of existance by writing directly over the top of it.
I guess the moral is designing these little scripts actually requires more care than I usually give them.