OK, this one's a little complicated, so I've included pictures. Dispensers which place water sometimes trigger when they shouldn't (when compared to the case where they shoot out any other item). That is, dispensers sometimes behave inconsistently depending on the item they contain.

To demonstrate this:
1. Build two dispensers on top of each other all facing in the same (horizontal) direction.
2. Build 3 vertical pillars 2 blocks high, which surround the space where the dispensers dispense (to create a 2 block high vertical tube with dispensers all on one side facing inwards - see picture).
3. Place at least one of any block/item that would normally be fired or dropped by a dispenser in each of the two dispensers.
4. Power the top (and only the top) dispenser.

Doing this, a block/item is fired out of the top dispenser only, whilst the bottom dispenser does nothing at all. Repeating step 4 does the same thing - it fires another item out of the top dispenser only. This is expected behaviour.

5. Now replace the contents of both dispensers so that they each contain a single water bucket.
6. Power the top (and only the top) dispenser in exactly the same way as done in step 4.

What I expected to happen was...:
for the dispensers to behave in an identical manner to the previous case (4), i.e. for a water source block to be placed by the top dispenser and for the bottom dispenser to do nothing. Repeating step 6 should then once again trigger the top dispenser only and remove the water source block at the top and leave the tube completely empty.

What actually happened was...:
the top dispenser triggered, placing a water source block as expected, but the bottom dispenser then also triggered after the top one, placing water in the bottom of the tube. Repeating step 6 then gives normal triggering behaviour - only the top dispenser triggers, which removes the water source block from the top, but then leaves a water source block at the bottom (picture 3).

This is entirely inconsistent behaviour and has broken several devices that act as boat elevators/lifts (which require water source blocks to float), which worked in previous versions.