I am currently casting a large structure out of obsidian in my survival world, and so I have handled hundreds of lava buckets in the last week or so. I started this project while running 1.4.7 and did not have this issue - I filled buckets with lava from a lava lake (around level 11-12) and poured out the lava source blocks where I wanted to create obsidian, with no problems.

What I expected to happen:
After upgrading to 1.6.1, I expected that lava buckets would still remain in my inventory until I chose to empty them.

What actually happens:
Since I upgraded to 1.6.1, I have been plagued by a bug where buckets of lava in my inventory are randomly replaced by empty buckets. Sometimes the bucket becomes empty moments after it is filled; sometimes it will become empty minutes later. I have not noticed buckets in chests becoming empty, just buckets in my inventory. Regardless of when the bucket empties itself, no lava source block appears anywhere; the source block seems to disappear/be wasted. I have not been able to identify anything that triggers this behaviour. Moving the buckets around in my inventory, filling buckets from a stack as opposed to individually, and distance from/position of source block seem to have no influence. At least 30 buckets of lava have randomly emptied themselves since I upgraded.

Steps to reproduce
Difficult to say, because this error seems to occur at random. What I'm doing:
1. Walk to lava source with five empty buckets and one water bucket in inventory (bottom/visible row), plus miscellaneous tools and stacks of dirt, stone, gravel, obsidian etc
2. Fill lava buckets from lava source.
3. Walk back to location of structure (~40 blocks)
About every fifth time I make this trip, at least one of my lava buckets will be empty by the time I get to my obsidian casting site. It seems to happen slightly more often when my route back to the site includes 6 blocks climbing up a ladder, but when I take the longer way with no ladder, it still happens.