If you repeatedly slide your mouse back and forth over the tags listed for a question (that is, toggling their hover status many times), the user box slides farther and farther to the right, stretching the question frame (as seen by the bottom border) and causing the box to go under the right-hand sidebar and eventually off the screen, causing horizontal scrollbars (ew).
Happens both main and meta. I am using Chrome 58.0.3029.110 (64-bit) on a Windows 7 computer.