If you get someone else's mail in your mail box and you accidentally open it and then realize that the mail is addressed to someone else then all you need to do is seal it back up with some tape and take it back to the post office and let them know that you got someone else's mail and accidentally opened it.
You will not get into trouble with the law or anything if it was just an accident and then the USPS will just return the mail to the sender or send it to the correct address.
It's happened to me before and it was a check from someone's job and it was addressed to my address but turns out it was meant for someone else that used to live at this address and the sender accidentally sent it to the old address instead of the new address the person now lives at.
I just put the check back in the envelope, sealed it up with some tape and took it back to the post office and told them and they just sent it back to the sender.