I see this on my flag-summary page:
At first I thought this was because not all pages on the site are responsive yet, but this one (otherwise) seems to be. The stuff on the main part of the page re-lays itself out as I change the window size, and there's no horizontal scroll. Here it is in a smaller window:
The flags at the top of my list are on deleted posts so at first I thought this was just about the pink background (which is sometimes way longer than the title), but here's that smaller window farther down. The red line indicates where the summary box (the right column) would start if it reached down that far:
Long flag text (like in the second screenshot) wraps; it's just the titles and the pink they bring along with them that's not minding the boundaries. I suspect that individual elements on that page have wrap directives and it got missed on titles.