...six kilos of worked Neolithic flint were found in a spoil heap because the excavators thought they were digging a medieval site!
...some charcoal found on a site was sent for radiocarbon dating. The response from the lab was 'This site is very, very, very, very old' - they had been sent coal!
...on a training excavation, a student had not found a single thing. Everyone felt sorry for him (yes, it was a him!) and so they buried a coin in his section. You've guessed it, he didn't find the coin!
...second year students from a well-known University came back from fieldwork with very strange results from levelling. They were using the staff upside-down!