If you have the gamerule dofiretick as false and set and Upper Half Slab on fire, the fire does not render, however the ash particles will and it will emit light.  When a player comes in contact with this invisible fire, they will catch fire.

To Recreate
1) Place Half Slab (of any type) to the Upper position.
2) Light on fire with Flint and Steel,
3) Have yourself or a player come in contact with the block.

See Attached photos for examples for no flames and only ash, and myself set aflame contemplating my decisions.

NOTE
This happens on Realms and Single-player worlds.

In the meantime, I promise not to have too much fun with this...