Title explains it. You can spawn a vehicle and enter it, and this results in you controlling both the vehicle and Mario at the same time - independently of each other, but using the same controls. Best way to fix this would probably just be to prevent Mario from using vehicles.

Or don't fix it at all because players shouldn't be doing this anyway. Idk.

Here is a link to a video that shows off the bug: https://youtu.be/GzuSFivTTok