Flowing water can fill cauldrons completely. 

How to reproduce:

1. Dig a 1 block deep hole in the ground
2. Place a cauldron in the hole.
3. Place water 5 blocks away from the hole containing the cauldron.
4. Wait for water to flow over the cauldron
5. Remove the original water source
6. Note that the cauldron still contains water

Observed result:
The cauldron is completely filled

Expected result:
The cauldron should not be filled