So, there's another one. The Penn Museum just found a 6,500 year old human skeleton...in the back of the storage rooms.
How do things like this happen? A lot of people with no background in archeology or museology wonder that.
People who've never had the privilege to spend time in the basement of the British museum. A fascinating place if you can get access to it - which is not easy. ('I'm writing a book' might get you a guided tour, though).
So, how does it happen?
Most of the "lost then found" artifacts were collected in the late 19th into the early 20th century. During that time it was fashionable for wealthy gentlemen (and even some ladies) to collect artifacts. They often donated them to the nearest museum. A lot of artifacts came in, some of them not well provenanced - remember, these were amateurs. And in many cases museums just put anything that wasn't pretty (like, say, skeletons) in boxes in storage to sort out later.
Then later never actually happened.
Museums have larger collections than what you see in glass cases and on shelves and sometimes they don't really know what they have.