Harry Potter and the Order of the Phoenix
Augustus Pye is a Trainee Healer at St. Mungo's when Mr. Weasley is recovering from the attack at the Department of Mysteries. He is, in Mr. Weasley's words, "a lovely young chap and very interested in ... um ... complimentary medicine ... I mean, some of these old Muggle remedies ... well, they're called stitches, Moll, and they work very well on – on Muggle wounds —" (23.132). So Augustus Pye is the one who is responsible for putting the stitches idea into Mr. Weasley's head. It doesn't appear to work too well for Mr. Weasley, and his argument with Mrs. Weasley about Muggle medicine is what drives Harry, Ron, Hermione, and Ginny out into St. Mungo's. Without stitches, they would never have seen either Professor Lockhart or Frank and Alice Longbottom, so in a weird way, Augustus Pye is the cause of a major plot point.