And then forgot to send test messages, or forgot to check the test messages, before signing off on the email campaign.

(Full disclosure: I used to work for a web-centric company on the technical side of communications, ensuring that our newsletters didn't get waylaid by spam filters, etc. We used many hundreds of test accounts across all of the major email services to validate our campaigns prior to enabling the full mailing. There are also companies that will do this for you, showing you how, exactly, the test message will appear at each email provider and/or browser.)