When sand (or gravel) falls in to lava, the falling entity appears to be on fire. The block will settle normally, it only briefly appears with the flames.

To reproduce:
Place sand above a block of lava (or a pit of any depth) and watch the falling entity as it first touches the lava.

To fix:
This is a small bug fix I wrote for myself using names determined by MCP. In EntityFallingSand add the following method:

public boolean isBurning()

Since the gravel and sand aren't flammable it seems strange to see them burn when using sand to fill in a lava pit.