I used to work sorting mail in a facility with 200 residents, plus staff (so decent amount of mail every day). If I'd had trust in USPS prior to that, I would have lost it.
Here's some of the best:
1) Envelope saying, "we damaged your mail," but the only thing inside is the scarp of envelope with the address on it.
2) Envelope saying, "we damaged your mail," with a magazine inside - which was addressed to somewhere else. So not only did they damage this guy's mail, they also sent it to the wrong place.
3) Mail from Kentucky (facility was in NY)
4) Postman dropped off stacks of mail intended for nearby apartment buildings by us.
5) Mail delivered to us by accident, we "returned to sender," came back to us. Even when we covered every possible identifying mark except the return address, it came back to us, so the only possible explanation was that the postman saw it in the outgoing mailbox, and just brought it back to us.