Probably the longer I think about it the dumber and dumber things I will remember, but one that comes to mind is unintentionally configuring a test suite to use

Probably the longer I think about it the dumber and dumber things I will remember, but one that comes to mind is unintentionally configuring a test suite to use a live SMTP connection instead of a mock one. When the tests ran, thousands of e-mails to invalid e-mail addresses tried to send for real, bounced, and temporarily affected the "reputation" score on the transactional e-mail service.