The worst is when UPS decides to use the Post Office for "final delivery".
I had UPS claiming that they had "delivered" the package to the Post Office who was supposed to complete the delivery. The Post Office, of course, would say "That's a UPS tracking number, we don't know anything about that."
Basically UPS and the Post Office point fingers at each other, and I'm left without my package. Fortunately, the item was only worth about 10 dollars. Never did get it though.