I found this out when I was playing in SSP. I had a full inventory from mining and had 4 stacked empty buckets, when I tried to get some lava from a underground lava lake I got the lava bucket but it fell out of my inventory and into the lava lake I now had 3 stacked empty buckets and a lava bucket that was now in the lava...
You can reproduce this bug by: having a full inventory and having some stacked empty buckets and then trying to get a bucket of lava.
I think this happens because your inventory is full so it has no where else to go.