Whenever the normally goody-two-shoes Heather decides to "show a little leg". And by a little I mean a lot.
Heather went to a Halloween party dressed as French maid. But all the extra attention unsettled her, so she ended up swapping costumes with Kevin.
One time Heather wore a dress with a very, very short hemline that she got from her mother's old wardrobe. Marsha, predictably, tries to stop it, saying, "I will not have my daughter wear a dress she can't bend over in!" Then George walks by and says, "Marsha, isn't that the dress you used to wear no underwear with?"