I noticed now that Stack Overflow has its padding set to
0, unlike before.
Note the padding on left side of the page; it looks shifted to the left and everything is touching the left-hand side of the screen.
Edit: Back to normal on mobile (iPhone 4 v6.1.3 / Safari).
Edit: Same problem on mobile full site version, mobile version is fine (iPhone 5 v6.1.4 / Safari).