My old professor told me to always check the spoil pile, and I'm glad I did

Back in my field school days, a professor named Dr. Carson gave me a piece of advice I thought was silly. We were excavating a test unit at a site in southern Utah, and he told me, 'Abby, always sift through the dirt from the spoil pile at the end of the day. You never know what got missed.' I figured our screening was careful enough. On the third day, tired and ready to quit, I almost skipped it. But I did a quick check anyway. My trowel hit something hard in that pile of what I thought was just backfill. It was a complete, tiny bone needle, about two inches long, that had slipped through our quarter-inch screen. That find helped date the occupation layer we were working on. It taught me that the job isn't done until you've double-checked everything, even the stuff you're sure is just dirt. Has anyone else had a simple piece of field advice turn up a surprise?