It's the first time I've noticed this on a SE site but I guess it applies not only to Apple.SE.
On pages where the contents load relatively slowly (e.g. sometimes, the "Users" page) there is a noticeable Flash of when the page is built, and at some point gets a vertical scroll bar. The page's contents move 16px to the left once the scroll bar is added.
It's a minor thing, but noticeable. I would consider using
overflow-y: auto in order to force a scroll bar, whether there is need for one or not. It would make for a smoother browsing experience.
Bumping this, as it still happens.