In certain areas, items (Potions, TNT/Tridents, Food, Arrows, Equipment) will fall through the floor. This only occurs in areas that are entered via doors, and does not always happen. It also doesn't seem to affect gold or emeralds.

So far, I have seen this occur in:

• Ancient hunt gold rooms
• Ancient mob arenas
• Radiant Ravine's "Tropical slime room"
• Probably other places, I just don't remember