Over the last 28 years, I have seen a lot of good direct mail and a lot of mistakes. Sometimes they have been sad, like when a nonprofit had the wrong return address on their courtesy reply envelopes. They did not get any of the donation checks delivered to them. Other times, they been really funny. Like the time when a wrong phone number was put on the mail piece so when recipients called it they reached a sex hotline. That was pretty funny. The worst mistakes are the ones that cost you the most money, so learning what not to do can really help.