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

1. Summon a command block minecart

   /summon commandblock_minecart
   

2. Put an item with right click action in your hand, for example a fishing rod
3. 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.