Anally raping her, no less; and in California, where Governor Newsom just signed a bill into law allowing judges to reduce the legal penalties for doing that. I'll bet Polanski's regretting having fled to Europe over that incident now. (He fled because contrary to his expectations when plea-bargaining, he was going to have to spend some time in prison, though it was still a pretty light sentence as I recall.) The legislature's fig-leaf justification for passing that bill was that the penalties for anally or orally sodomizing an underage child (of either sex) were higher than for vaginal rape, so they were equalizing them; it didn't occur to anybody to try raising the penalties for vaginal rape instead?
Incidentally, something a lot of people don't seem to have noticed in the transcript of his victim's heartbreaking testimony: the lawyer asks her whether she knows what sex is, and whether she's had any before, and she answers both questions in the affirmative. She was thirteen at the time. If that's how things were in California then (when it could still give its electoral votes to Ronald Reagan in 1980 and 1984), I'd probably hate to see how much further along on its slide into depravity the state is now when it's a one-party Democrat-run hellhole.
There was some shadiness surround Polanski's case. He pled guilty to a lesser charge, the more serious ones were dismissed, and he was ordered to spend 90 days in the mental hospital in Chino Prison. He served around 40 days and was released. After he was released, the judge threw out the plea bargain and re-sentenced Polanski to 50 years in prison. At this, he fled the country.