What would happen if you out the wrong heart into someone's body?

I suppose it just wouldn't go in. The heart 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, or the heart can't be entered at all.
2. Placing the wrong heart kills the person, and possibly also the person whose heart it is.
3. Having the wrong heart placed back in you could give you some kind of identity disorder, or give you 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.