If there is lava above a block, and you position yourself with your head directly beneath the block, the lava overlay appears as if you were in lava, despite a block separating you.

In the attached screenshots, all I did was take a step forward.

To reproduce:
1. Place any block two blocks above the ground
2. Place a lava source block above it, and blocks around that to contain the lava
3. Stand under the block