More info for noobs: The new self checkouts have handheld scanners. If you use the scanner, the register will assume you're not putting it on the scale, so it will ignore the weight. For instance, if you shove several items into the hand grip hole on a box of a larger item, then ring it up this way, the register has no way of knowing that the box weighs more than it should. Go home, remove your shit, use a hot glue gun to reseal the box, and return the big item.
Tips for the "Obvious" folder - Pay cash. And for fuck's sake, don't let any people or cameras catch you shoving shit into the box.