Here is one that was missing in action for over a year and a half. I don't know whether it was lost in the U.S. postal system or that of Spain. Maybe someone can figure out the markings on the back for a better clue.
I sent this envelope containing the two Spanish imperforate souvenir sheets for the 1958 Brussels World's Fair (Scott Spain #877a-878a) to a collector in Spain who had purchased them on eBay. Several weeks later, she contacted me to advise she had not received the shipment, so I issued a refund.
The package was mailed on January 11, 2011. The only dated markings indicate it was then handled by the Spanish postal authorities on August 28, 2012. By that time, the addressee had moved and the package was subsequently returned to me in September of 2012.
I e-mailed the purchaser to advise her the package had been returned to me. To my surprise, she still wanted the items! She re-sent payment and I re-sent the package, which she received this time.
Where it was for a year and a half, I'll never know.