Don't throw the ball too far if your dog is fastened to your belt with a retractable leash. Especially if you are not looking when the dog reaches the end of the leash at 25 mph, and you are taking a step and one foot is off the ground at that instant.
Don't lean your favorite 16 lb bike against a railing on a big bridge on a very windy day.
Don't try to walk in a low ceiling basement with overhead pipes while wearing a hat with a visor. You can't see the pipes.
Don't let your friend keep his motorcycle in your garage and his truck in the driveway when he goes to interbike, if you know he leaves stuff all over the place and never takes it back, but you want to be nice to him. Even if he does own a bike shop you can see from your house.
Don't go outside with the dog in the nice weather every time you let him out, no matter how much you like it, unless you want to go out in the winter too.
Don't be too nice to all the wierdos. Don't let them know where your house is.
Don't send a cashiers check to a stranger for an ebay purchase.
Don't waste money on a brand new car, it's a huge waste of money.
Don't always buy the cheapest of everything. Sometimes that ends up costing more.