Dispensers with bucket don't work on cauldrons.
I had an dispenser with a bucket pointed to a cauldron filled with water. I expected the bucket to fill after the dispenser was powered, but the bucket was dropped as an item into the cauldron.
Steps to Reproduce:
1. fill a cauldron (with water or lava)
2. have a dispenser containing a bucket pointing to it.
3. power the dispenser.