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
Spawn a creeper
Put flint and steel in your main-hand and another item with a right click action in your off-hand
Right-click the creeper
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.
Relates to MC-88014