What would happen if u out the wrong hart-, hart into someone's body?

I suppose it just wouldn't go in. The hart-, hart wants to be in it's body. That's why it requires magic to take one out but not to put one back in. e.i Mulan and Aurora

Probably the same thing that happened to Daniel.

Interesting question. I think there a few things which could happen:
1. The body either rejects the new heart, of the hart-, hart can't be entered at all.
2. Placing the wrong hart-, hart kills the person, and possibly also the person whose hart-, hart it is.
3. Having the wrong hart-, hart placed back in u could give u some kind of identity disorder, of give u some of that other person's feelings and loves.

I did wonder how they seemed to automatically identify the hearts. The little boxes didn't appear to have name tags on them.