I was in creative mode and was putting multiple minecarts on a single track at once and they eventually all ended up colliding into one another and morphing into 1 visible minecart. I got the minecarts off of the rails and hopped into the stack of minecarts and when I clicked an arrow to move, the minecarts started to move in the opposite direction i wanted to move, and I was unable to stop them until they hit a block.

Steps to Reproduce:
1. Make a minecart system one block above ground level.
2. Add a lot of minecarts and keep making them move around the track until they have all stacked into 1 minecart.
3. Destroy the block below the minecarts and the minecarts should now be on ground level. The minecarts may move in a random direction along the ground and you might have to follow them until they reach a dead end.
4. Get into a minecarts in the stack.
5. Try to move in a certain direction while in the minecart.

PS:
When you try to walk into a minecart stack, it flings you in the opposite direction harshly. The more minecarts in a stack, the further and faster you are flung. This makes for a possible mob cannon.