When a water or lava source or flowing block above your head, and there is a block (it does not matter transparent or solid) between your head and the lava/water it seems as if you are under water/lava. This only happens when there is lava/water over a block that is right above your head.

This first occurs in 14w06a it does not happen in 1.7.4
It occurs in all gamemodes including spectator. No damage is dealt in survival or adventure.

To Reproduce:
1) Build a structure with ceiling 2 blocks above the floor (so you can barely fit under it)
2) Fill the top with water/lava (make sure you put sides to hold the lava )
3) Walk under it (it should look like it would if you head was really underwater)