What would happen if あなた out the wrong ハート, 心 into someone's body?

I suppose it just wouldn't go in. The ハート, 心 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 ムーラン 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, または the ハート, 心 can't be entered at all.
2. Placing the wrong ハート, 心 kills the person, and possibly also the person whose ハート, 心 it is.
3. Having the wrong ハート, 心 placed back in あなた could give あなた some kind of identity disorder, または give あなた 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 タグ on them.