As mentioned here before, John Roberts clerked for William Rehnquist. While Roberts would have been the first person to serve on the Supreme Court alongside a judge he clerked for, he will now be the first person to take the seat of a judge he clerked for.
Rehnquist himself clerked for Justice Robert Jackson. Interestingly, Rehnquist was the first justice since Justice Jackson to _____.
(I imagine there are a lot of correct answers that fit there, several of which are probably interesting. I have one in mind.)