Lady Bracknell's Achilles' heel
It will smart so much, in fact, that they may find themselves uttering phrases such as, "You, sir, are no longer my friend!" and "Don't expect a Christmas box from Bracknell Towers this year!".
Fortunately, the osteopathic gentleman has grown accustomed over the years to the sort of threats which Lady Bracknell utters when the treatment is more than ordinarily painful. He knows that, although she speaks in earnest at the time, she has too much faith in him, and owes too much to his skills, ever to voluntarily remove her name from his list of patients.