This is a bug I found in the new release (0.7.something).
I had 10 mB of melon juice in my left tank on my melon backpack. I wanted it completely empty, so I tried to put a bucket in the bucket slot to see if it would glitch and empty it. It kind of backfired: the bucket completely disappeared to never be seen again, instead of just staying there empty.

Fixed in the HUGE COMMIT OF EPIC GLORY