[MegaBlues]

To be more specific, they seem to be nodes which vanish after use that can be potentially gathered from multiple times.

http://imgur.com/a/O3NLC (Respectively, a chest full of gold, a Forest-based medicinal plant, and a wooden node. I'm sure this problem also happens with ore nodes in dungeons which vanish after use, but I don't have a mining character.)

Chests which give multiple items at once, medicinal plants which don't vanish after use, and the Stolen Good caches/Goblin caches don't seem to have this issue, or at the very least, I wasn't able to trigger this.

EDIT: I just made this happen with a chest, so add that to the list. It was in a BDP, and it was trapped.