This must be a bug that's already reported, but I can't find it.

When I break cactus, sometimes the block becomes invisible on the ground, but most often it totally disappears.

It seems more likely if the drop lands on the part of the cactus still on the ground, as if the "collect several items in one pile" kicks in even though it shouldn't.

Example: http://youtu.be/eUisTg0Dj7o

1st sequence shows cactus block completely gone
2nd sequence shows cactus blocks becoming invisible but still possible to be picked up from the ground.

(Don't know if it's related to the issue Etho had in UHC s16 e3 1:05 https://www.youtube.com/watch?v=creeCiAGMR4 into the episode, where his crafting table disappeared as a pig walked in as he was placing it.)