I once got misdelivered mail that was destined for somewhere about 1,000 miles and about 5 states away. I had never considered that the envelopes stuck together, which might explain it, though they were not stuck together when I got them. I was very puzzled since there were no similarities in street name, address, city, state, zip code, etc...
On the flip side of this poll, I'm always impressed when the post office correctly delivers mis-addressed mail. I currently live in a large complex with several hundred units, and mailboxes spread throughout (about one group of mailboxes outside of every 4 units, and no labels with occupant names). Sometimes either we forget the unit number, or somebody's software chops it off when printing the address. Yet the mailman has figured out the correct unit and put it in my mailbox. Even when it's addressed to my wife's maiden name, and there are no other pieces of mail with that name.
The USPS isn't anywhere near perfect (I'm usually disappointed by their package redelivery in my area, for example), but there are many employees that take their jobs seriously and do a fantastic job - and to them, I say kudos. And I hope their enthusiasm about their jobs spreads to more employees...