I've mislaid some stuff I'd rather not have over the years, but never this badly. Karen Nelson, an archivist for the Lawrence Berkeley National Lab in California, discovered 20 vials of moon dust from the Apollo 11 flight stashed away in the lab's warehouse.
The vials had been sent out to the lab shortly after Apollo 11 returned, and bear handwritten labels dated "24 July 1970"; but once the experiments were conducted, they weren't sent back to NASA, as they should have been. Instead, they found themselves vacuum sealed in a glass jar and left to gather dust for the next four decades.
Berkeley Lab's Julie Chao adds:
Nelson contacted the Space Sciences Laboratory. “They were surprised we had the samples,” she said. She then contacted NASA, who asked that the samples be sent back but allowed her to first open the jar to remove the vials.
Berkeley Lab archivist Karen Nelson holds lunar samples used by Melvin Calvin for scientific experiments 43 years ago. (Photo by Roy Kaltschmidt)
Interestingly, NASA knew that the samples were missing; Space.com reports  that of the 382Kg brought back from the moon between 1969 and 1972, very little is unaccounted for:
Of the 68-gram batch of lunar material distributed to Calvin and his collaborators in 1970, NASA knew that only 50 grams was returned, said Ryan Zeigler, NASA's Apollo sample curator at the Johnson Space Center in Houston.
Space agency officials assumed that the unaccounted-for 18 grams had been destroyed during testing. Zeigler thinks the rediscovered, roughly 3-gram sample likely ended up in storage as a result of some miscommunication.
The dust had apparently been used for a paper assessing the carbon content of lunar samples  as part of NASA's search for extraterrestrial life. As you may already know, they didn't find any. It was a disappointment.