1. The store clerk undercharges you for an item you purchase? Considering that almost all stores use a computerised UPC scanner to ring up sales (even small "ma & pa" markets), I would be pleased to discover that the item had been on sale!
2. The cashier gives you change for a fifty dollar bill when you only gave her a twenty, and you don't realize it until you're out the door? There's no question about it, I would go back and return the thirty dollars. I would feel terrible if I kept it after discovering the error.
3. You see your best friend's husband/wife with another woman/man at a neighborhood restaurant? I would stop by the table and say hello. There are plenty of innocent explainations for this situation. The other person could easily be a friend, relative, or business associate who I do not know. If I discover this is not the case, I will keep quiet about the situation unless asked by my friend.
4. Your 15 year old asks you if you ever tried an alcoholic beverage before you were of legeal drinking age (and you have)? I'd be honest. Lies are always exposed in the end, even the ones you tell your children. Besides, this could open the door for some good discussion.
5. You find a one hundred dollar bill laying on the floor of a department store? If I saw who had dropped it, I would definitely return it. If not, I would inquire with anyone nearby to see if anyone was missing some money. If there was no one around, or no one missing the cash, I would keep it and smile over my good fortune, since I had done everything that I could to return it.