1. Enter a dark area
2. Light it with a temporary source of light, such as a torch or amulet of light
3. Drop an item that's on your autopickup list (in my case, I used potions)
4. Walk over it again to pick it up
5. The message "You pick up the -foo-" is generated, and the object appears in inventory, but on the ground the object still appears.
6. Darkening the room and re-lighting it causes the ghost item to go away

This does not seem to occur for items that were not automatically picked up, and doesn't happen in the dark.