Years ago I came across a fellow pilot who had about 1,000hrs of H369 tuna spotting time written in his logbook, but probably wouldn't have been able to tell you on which side you're supposed to sit when flying one. We figured he had been sitting in the harbour in Guam and just copying the registration numbers of the machines that came through on the fishing boats, and since those things usually don't have much in terms of real flight records, or any legitimate paperwork for that matter, you couldn't really proof he made it all up.
Brings up the question, what would you write in the aircraft ID column if you wanted to log time in an unregistered aircraft?