I had a new one to me at a JV game week 1. Luckily someone on the crew knew the rule when I did not.
On the try Team A lined up in a kick formation. Holder takes the snap, stands up, and sprints towards the pylon. As the L, I move from LOS to the goal line. The ball carrier is hit at the 1 yard line, ball pops loose and flies into the end zone where it is caught by another Team A player.
I rule a successful try. H comes out from Team B's sideline with a "not so fast my friend." I was unaware the 4th down fumble rule applied also to a try down.