I opened a package that was mistakenly delivered to my house.  I was
expecting a package and realized my mistake after opening the box.  I
planned to return it to the correct address that same evening.  Later
someone knocked at the door.  It was the intended recipient.  He had
learned through FEDEX that the package had been delivered to our
address.  He took the package and returned later when he discovered
that a battery had not been included.  There was no battery in the box
when I opened it.  He was upset that the package had been opened and
the battery was not included.  He and his friend mentioned reporting
it to the authorities.  I apologized and explained that it was an
accident and that the battery was never included in the package.  He
didn't accept the explaination and left very upset.

Since I mistakenly opened the package delivered to my house (FEDEX and
not US mail) did I break any laws?  What is the law in this case?

In my opinion, you didn't do anything wrong and the guy was wrong for
coming to your house. I had this happen with FedEx last year-- I was
tracking a package and one day it said "delivered" and had some random
guy's signature. They delivered it to the right street name but to a
town 45 minutes away.  We called FedEx the next day and they had to
make a special trip to get it.  That's what this guy should have done
rather than invading your privacy like that by showing up at your
door!

For him to assume that you took his battery-- when it was obvious to
everyone and even documented online that the package had been
delivered to your house-- is ludicrous. You'd have to be a really dumb
criminal to do that.  Most people would assume that the company was to
blame, and also be upset with Fedex for making the wrong delivery.  He
seems like an inconsiderate clod and I wouldn't worry about him for a
minute more of my life.  That's my 2 cents on the situation :)

---

Clarification of Answer by keystroke-ga on 10 Jul 2006 22:07 PDT

That was supposed to be a comment! But anyway, since I accidentally
took the question, here's the legal aspects of it..

It is a federal crime to open mail that is not addressed to you. 
However, you did not realize that you were doing so and you did not do
so with a fraudulent intent so you would be in the clear.

Here is the statute (18 U.S.C. 1702) regulating this:

Section 1702. Obstruction of correspondence

      Whoever takes any letter, postal card, or package out of any post
    office or any authorized depository for mail matter, or from any
    letter or mail carrier, or which has been in any post office or
    authorized depository, or in the custody of any letter or mail
    carrier, before it has been delivered to the person to whom it was
    directed, with design to obstruct the correspondence, or to pry
    into the business or secrets of another, or opens, secretes,
    embezzles, or destroys the same, shall be fined under this title or
    imprisoned not more than five years, or both. 

Sources:
misc.legal

Since you had none of those intentions, and furthermore since you did
not take it from a mailbox or post office, you don't violate the law.

I hope that helps and if you need any more help, let me know.

Cheers,
--keystroke-ga

Thanks.