A classic case of mis-cataloging resulted in the skeleton and skin of the last known thylacine being "lost" in the museum cabinets. This isn't quite as uncommon as you might think; museums have a lot of items donated and as cataloging systems are updated and buildings are updated and specimens are moved around (not to mention the change in an item's identification as more samples are found (classically, mis-identified dinosaur bits), things get forgotten or "lost."
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-- Byrd
Moderator, Hall of Ma'at