If a skeleton shoots an arrow through some lava/fire or uses a flame bow, the flaming arrow can explode TNT even if /gamerule mobgreifing is set to false.

How to reproduce:

1. Spawn in a skeleton and trap it.
2. Place a block of TNT nearby
3. Place some lava in between the skeleton and the TNT
4. Switch to survival and stand near the TNT until the skeleton hits it.