This happened to me once.
A friend (not particularly close, though) was getting married. She was good friends with one of my good friends. He told me when was the wedding, so I went to the ceremony with no intentions whatsoever of attending the reception.
After the ceremony ended, the happy couple announced that everybody present at the ceremony was welcome to attend the reception.
Not only did I attend the reception...but there was so much cake left over, that the bride told me to take it home, since I knew I lived with my parents, brother, and two dogs.
So I do not know how many faux passes did I end up committing, albeit unintentionally.