Always check your bank statements people!

So Brian and I went to dinner the other night. The whole experience was pretty good. Then this morning, I decided to check our bank account online and found that we actually paid for someone else's meal as well as ours. After calling the restaurant and having the manager do some investigating, we come to find out that the waitress must have been running several cards at a time and she ran our card for 2 different tables. What did the people at the other table do when they received a receipt ready for a signature that had SOMEONE ELSE's name on it? Did they overlook that it wasn't their name on the receipt and just sign their own? No. Did they notice a different name and tell the waitress that they had someone else's receipt? No. They SIGNED BRIAN'S NAME ON THE RECEIPT. It really surprizes me that people do this sort of thing. I just don't understand it.