And I can't post in the bug thread, says I lack permissions.
I currently use the second to last hexxit update (the one that ruined disenchanting, but still has the awesome shears)
The bug occurs during some combination of dropping items and storing items. Usually I lose my fancy tools due to this bug (they just vaporize) but this time I watched it happen to a capsule that I had stored my creation in.
So:
1) Stored capsule in backpack
2) Noticed annoying sand floating around me (due to magnetism) and placed sand in bag.
3) Capsule that I had placed in backpack was now in my hand (WTF?) placed capsule on sand and threw the sand away.
Sand remained, capsule vaporized and was not in the backpack I had put it in.... therefore losing hours of time designing, creating, and redstoning my tower.