0x7fffffff is a member of Motor Vehicle Maintenance & Repair with the site in their top five sites, and also happens to have 12 digits for the numerical values for their Stack Overflow account (5 for reputation, 2 for gold badges, 2 for silver badges and 3 for bronze badges). This causes the design of the Accounts section of the user page to break on some sites.
The example above is Ask Different, but this is also broken on the following sites: Electrical Engineering, Theoretical Computer Science, TeX, Mathematics, Home Improvement, and English Language & Usage. Shown below is the accounts section of the profile page of the user on the above sites. This problem may occur on more sites, but these are the sites that the user is a member of at the time of writing where this problem occurs.
Removing any of the digits fixes the design, adding slightly more padding between the 'r' of 'Repair' and the reputation and the line break on 'rep' and the number of bronze badges.
This looks fine on Stack Overflow and other sites where the text size is smaller.
OS X 10.9 with Safari 7.0.6 (and 10.10 with Safari 8.0 beta FWIW—I know beta isn't supported). Normal 100% zoom level. Retina display shown above, but also reproduced on non-Retina.