[Prefatory Note: I wish I could take credit for this tip, but I cannot. Thus, this tip comes from a loyal reader]
Sometimes, you might eat at a restaurant where the cost of the meal is discounted for some reason, e.g., the restaurant has made available coupons that entitle you to a discounted price. After the meal, when the check comes, you calculate and write-in or leave a proper tip, but, not thinking, you reflexively base the tip on the discounted price. When you get home, it hits you like a sledge hammer when you realize what you've done. Before driving back to the restaurant, try calling the restaurant, because there's a good chance you can add the enhanced tip amount over the phone, after which you will feel better and you will be assured your server is very appreciative.