I once had my wallet go missing from a tray at EWR, and noticed once I was in the AC. TSA were very helpful, re-scanned my possessions to see if I'd simply mislaid it about my person, but it wasn't there. I was in a blind panic for a good half hour before it turned up, having been knocked out of the tray and down the other side of the table.

Going back to the secure side, I was careful to put my stuff in the tray exactly as it was before, and pushed my items through. Sure enough, there it went again, knocked out by the dangly things that stop all and sundry getting ionised swinging back and knocking it out.

I make sure to pile everything inverse now, loose stuff at the bottom, coat on top. I must say how brilliantly helpful the TSA that day were at relocating said item though. I even wrote a letter.