This just happened a short while back and you have my word that it's true.
Back in 1995, I had loaned Guy Dunscomb my SP Quarterly Locomotive Assignment Books for a project that he was working on. Guy was always really good about returning things in a timely manner and I got them all back in a USPS cardboard box that was a perfect fit. Because of this, I have kept them in there for the last 16 years and have had reason to use them again many times, perhaps over 50. Each time, I tip the box upside down and gently let the booklets fall out onto my table as they are hard to get out any other way.
Well, I did this again for the umpteenth time and happened to see a couple of slides fall out too. There was a Carousel on the table that I was going through at the same time, but it was full, so I looked at the two slides. Both had Guy L. Dunscomb stamped on the cardboard mount.
One was a 1955 UP steam passenger train in the snow and the other was taken in 1969 of a UP passenger train in Utah. They had been in there for 16 years underneath one of the cardboard flaps on the bottom of the box.
Years earlier, Guy had given me a few of his slides as thanks for something that I'd passed along to him. Now I wonder if he just didn't do this again?
The moral here is to always carefully check everything when you receive something by mail.