I once was a dispatcher for a contract mail trucking company. Part of my orientation was to tour a sorting facility and ride a few of the routes. I was aghast during the tour. It reminded me of an Escher drawing in action. There were conveyor belts all over the place and the worst was all of the envelopes on the floor that had fallen off the belts. It actually is amazing that packages get to their intended destinations. BUT.... I did learn not to use envelopes. The larger the package, the harder it is for it to get lost. I've stopped using envelopes even for a letter. I'll get the free priority boxes and use those even though it costs more, the peace of mind is worth it. Never had one lost. You can't beat the prices though and if things DO work right, I've had packages make it to CA from FL in 2 days. That's not to shabby.
When you see all of the trailers in the bays at the centers, it is no wonder that packages sometimes take "detours" on their way to their destination. It is so easy to put a package in the wrong bin and have it go on a wild goose chase. The craziest mix ups occur when doubles are used. If they get the trailers mixed up (which happens frequently) that whole trailer of mail can get lost for a while until they figure out the trailer number and put a trace on it. We found one trailer full of mail parked in a storage lot.
If you ever have the chance to tour a sorting facility, take it just for the snickers. It definitely proves that Murphy's Law is alive and well.