alert emails
On three different occasions, with three different developers, I've had the pleasure of trying to recover and clean a mail server after they've written a try..catch in their code which sent any errors to the company alert email address - but didn't put any logic in to see whether the email had already been sent.
The record was 176,000 emails sitting in the queue of the local environment MTA, waiting to be sent to our main email server after the developer set a site running on a Friday afternoon, and I didn't get a warning until Sunday night. One email a second for over 48 hours, dammit!