I'm not even sure what kind of point you're trying to convey here. What you're describing is called "fraud" and is super illegal. If someone leaves their door unlocked, you could also enter their house and steal tens of thousands of dollars from them. Your point is...?

Yeah, this is just like a forged signature: falsified evidence of an agreement that never took place.

Yes, its a vulnerability, its also a serious legal wrong in its own right (on top of the trespass to chattels you need to do to accomplish it.)