Comment by joren-

To share my mistakes as well:

After launching a web app we noticed severe performance issues and had to roll back to a previous version. To identify the performance problems introduced in the new version, I wrote a script mimicking a thousands of typical user interactions. This also included sending email. In the moment, we forgot to properly configure the dev environment and the emails went out.. but it did not stop there.

This is a good decade ago and some clients still preferred to get faxes instead of emails. We used a service which automatically converted emails to fax messages. We tried to intervene but were not able to stop thousands of fax messages and basically DDOS'd many fax machines and then had to call the affected clients..