Fire that is on top of all none-wooden slabs, will be invisible. The fire still crackles, makes smoke, and burns players/items/animals etc. Fire is visible when on top of wooden half slabs.
Using the default texture pack for minecraft.
How to reproduce:
1. Get a row of various top half slabs, including stone, pur pur, wooden, etc etc
2. Lit all the tops on fire using a flint and steal, you will see only the wooden ones have fire
3. Stand on top of the none wooden ones and you will get burnt, proving they have fire on them.