Yesterday, I was detecting in a cow pasture, amidst a herd of cattle, hunting for some 19th century building sites. Sweeping over a dry looking cow-pie, got an iron grunt. Figured I ought to see if that was a square nail, so I flipped the cow-pie over with my shovel. Oops! It was still juicy on the bottom. No matter, I scraped the remains off the target, swept again, nothing! Another sweep, more nothing! Next, I swept over the cow-pie, bingo! One more pie sweep, bingo again! Still don't know what the target was, iron's not enough to make a dookey-digger of me.
(Incidentally, cows are great detecting companions. You can unload all your troubles on them, and they never judge, argue, or rat you out.)