What would happen if 你 out the wrong 心 into someone's body?

I suppose it just wouldn't go in. The 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 花木兰 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, 或者 the 心 can't be entered at all.
2. Placing the wrong 心 kills the person, and possibly also the person whose 心 it is.
3. Having the wrong 心 placed back in 你 could give 你 some kind of identity disorder, 或者 give 你 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 标签 on them.