The bug
When you right click a command block minecart while holding an item in your hand, it opens the command block GUI and uses the item.
How to reproduce
Summon a command block minecart
/summon commandblock_minecartPut an item with right click action in your hand, for example a fishing rod
Right click the command block minecart
→ You will see that the GUI opened and the item was used
Code analysis
Based on 1.11.2 decompiled using MCP 9.35 rc1
The method net.minecraft.entity.item.EntityMinecartCommandBlock.processInitialInteract(EntityPlayer, EnumHand) always returns false. Instead it should return the value the method net.minecraft.tileentity.CommandBlockBaseLogic.tryOpenEditCommandBlock(EntityPlayer) returned.
Appears to be fixed as os snapshot 17w43a.