Make a wide inverted slabs floor (single slabs) over solid blocks. Essentially, there's an empty space half a block high right under the slabs.
When items fall onto that floor, sometimes they "glitch through" and you have to break through your floor in order to get the items back.
This actually can occur with several of block types, not just inverted slabs.
Comments 2
Ok, sorry my description was wrong here.
#1) It's actually something that happened in 1.3.4 maybe 1.4.7. So maybe it got fixed indirectly through a version change or something.
#2) Seems to happen a lot more near walls and corners, especially from items "falling out of" a crafting table or broken furnace, than right in the middle of the floor. I don't remember if it happened to items deliberately dropped by the player, but I remember it was mostly items falling from breaking stuff.
#3) It seemed to occur only infrequenly. Sometimes not at all. Mostly, near edges. So it greatly depends on the configuration of nearby blocks.
Just close this one bug report here and IF I can see it again, then I'll document it better the next time.
Thanks.
Unable to reproduce. All items stayed ontop of the upper slab floor.