Ouch! that hurts. I think we all have those stories...
I remember installing 440 Six Pack motor back in a Cuda. The last thing I had to do was reinstall the distributor and fire her up. I placed the small 1/2 box end wrench near the open distributor hole in the block so it would be nearby as I placed the distributor in the engine. I had a bad feeling as I approached the engine with distributor in hand. I still swear to this day that I never touched the car or the wrench but I saw it just jump into the open distributor hole in the block and I heard clink, clank, clunk as it found its way all the way down into the oil pan! Of course there's that second or two that you think, hmmm, maybe I could just leave it there and nothing will happen. Yeah right. I had to pull the engine back out to remove the pan so I could get my arm in there and pull that wrench out...and then throw it about 200 yards into a field.