A Dispenser (with an empty Bucket in it) cannot suck the water from a waterlogged block that contains Kelp or Sea Grass in it. It spits out the Bucket instead.

EDIT: It seems like they can suck the water from Corals. They cannot do it with Sea Grass or Kelp though.

How to produce this bug:

1. Place down a Dispenser with an empty Bucket in it.
2. In front of the Dispenser place water and inside place Sea Grass or Kelp.
3. Enable the Dispenser.
4. Notice that it spits out the Empty Bucket instead of sucking the water in it.

Redo it with a Coral inside the water source and you'll notice that it sucks the water normally.