I live in an apartment building and it's pretty common to see envelopes laying on the table next to the mailboxes because they were placed by the letter carrier into the wrong one. If I am on the way back to my apartment and one of those is on my floor, I'll take it with me and place it in front of the apartment door, as others have done with me over the years.
It is fairly common to see or get one for another building in our apartment complex, even though my street name is different from the other 3 buildings. Same for another building nearby on a different street. But the letter carriers had the right apartment number!
We often get our mail delivered as late as 6:30 PM, another frequent complaint.
Another strange thing was when the letter carrier closed the big swinging door they open when they place the incoming mail into all the residents' mailboxes. Once, when they closed it, it trapped one of my letters between the big door and shelf of my mailbox, forcing me to either tear the envelope to retrieve it (or most of it), or wait until the next day when hopefully it would become dislodged. I thought of leaving a note as a reminder. Luckily, I saw it was a piece of junk mail (first time I ever rooted for junk mail LOL!) so tearing it was no big deal. The next day, the remaining piece of the envelope and its contents were sitting in my mailbox, dislodged. A few days later, I mentioned it to the letter carrier whom I happened to see. I knew she wasn't my regular carrier (she delivers to my ladyfriend's place nearby) and hadn't delivered mail to my building a long time. She expressed surprise.
But the oddest thing happened a few months ago. We have a locked box for outgoing mail built into the wall next to our mailboxes. The letter carrier has a key. One day, I went to get my mail and saw the door open but the box's dead bolt in the locked position, so the door could not close. It was basically swinging open. I had a letter to mail, so I just left it in the box. (Before the PO installed it back in the 1990s, residents just left their outgoing mail on the table, nobody was going to steal it.) Still, it's a pretty stupid mistake for the letter carrier to make.
But imagine my surprise when, the next day, after expecting to see the box closed and locked, it was still open with the door's dead bolt locked, and with the outgoing mail still in it! And we had gotten that day's mail delivered, so the stupid letter carrier failed to even retrieve the few letters including mine now stacked inside this still-open box. I was on my way out this time, so I gathered up all the letters in there and deposited them into a mailbox along my walk to my nearby ladyfriend's place. (She has had her share of issues with the same PO.)
When I returned home the next morning, I saw one of my neighbors, who happens to be on the co-op board, at the mailbox area. He already called the local PO and I told him I had delivered the letters (one was his). He was grateful.
That night, when I went to get the mail again, the outgoing box was closed and locked, finally. So we were back to our usual wrongly delivered letters to neighbors and other nearby buildings.
None of my monthly bills I mail myself any more. The PO has lost my co-op maintenance check a few times before I switched to having my bank mail it out. The PO lost my car insurance check in 2016 but I realized it in time to get them the money (I brought a new one to my agent's office). I now pay them on line. I buy a book of stamps maybe once a year now. No wonder they are losing money.