If something at a yard sale is far more valuable than the posted price, do I have to let the seller know?

If someone tells an offensive joke, is it my responsibility to speak up about it?

I worked 4 hours to complete a project. Is it unethical to bill the client for 8 hours?

My boss asked me to cover for him on his expense report by saying I was at a meal when I wasn’t. Is it unethical for me to do it?

^{You exit the grocery store after buying $100 worth of groceries. You look at the receipt and notice that the cashier gave you $20 too much change. Is it unethical to take the money?}

^{You are the captain, responsible for your passengers. If you are in a sinking life boat with the capacity for only 6 passengers, would you remove 1 passenger (yourself) to avoid the life boat from sinking?}

If you knew your friend’s partner was cheating on him or her, would you tell your friend?