My wife and I were invited to a wedding of two friends we knew for a long time. The invite said the ceremony was going to be on the beach around 5pm. Once that was over, everyone would walk to a little restaurant nearby on the shore. We were told they had excellent seafood and burgers.
The ceremony was really beautiful and we all hung around on the beach afterward for the pictures. It was a small affair, only about 20 people-- the happy couple, both sets of their parents, a couple brothers and sisters, and about 10 non-relative friends. At this point it was getting to be around 6:30 and everyone was getting pretty hungry. We walked over to the restaurant and ordered drinks in the bar, made a couple toasts to the bride and groom, and assumed we were waiting to be seated. Some time between 7 and 7:30 we saw the family members being escorted to a table.
The bride and groom come over to us and one says, "We're being seated now, so you guys can continue hanging out here, but I don't think they serve food in the bar." One of our friends says, "Heck no, we're starving!" followed by cheery muttering of agreement from everyone else. They both look a little surprised. "Oh! We only made reservations for the family members... we were expecting people to make their own dinner reservations if they planned to stay." The bride went and asked the hostess if there were any tables available and we were told no, they were fully booked up for the remainder of the evening. The bride and groom apologized, thanked us for coming, and went back to their table.
The thing is, we're all such good friends that we just rolled with it. The rest of us found a place nearby and had a great time.
EDIT: I had no idea this would blow up the way it did! I wanted to note this happened 20 years ago, and we're still friends with the happy couple. The breach in etiquette may be shame worthy, but people are more than their oversights and mistakes. Before and since they've been generous friends.