The bug

When holding a flint and steel in your main-hand and another item with a right click action in your off-hand, both items will be used when right clicking a creeper to ignite it.

How to reproduce

1. Spawn a creeper
2. Put flint and steel in your main-hand and another item with a right click action in your off-hand
3. Right-click the creeper
4. Note that the creeper was ignited but the item in your off-hand was used as well

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

For the client the method net.minecraft.entity.monster.EntityCreeper.processInteract(EntityPlayer, EnumHand) does not return when you right click a creeper with a flint and steel. Therefor it tries to use the item in the offhand instead.