What I expected to happen was...:
I place lilyPad over the water (2 block deep)

What actually happened was...:
I manage to place lilypad on the water level below, and then the water fills the hole and make the lilypad pop out.

Steps to Reproduce:
1. Create a water area 2 blocks deep
2. Point at a block near the edge of the water area with a lilypad in hand
->if you point and click on the half of the block nearest the center of the water area: the lilypad will be put correctly
->if you point and click on the half of the block nearest the edge : the lilypad will be put on the water below and then pops out by the water filling the hole

I have a poor english (you can correct my phrasing)

A last picture : if you put your lilypad (after having removed this one) above purple line->strange behaviour, below->normal behaviour.