Here is an example (using Chrome on Mac):
closed as too localized by Asad Saeeduddin, animuson♦, Matt, Toon Krijthe, Lance Roberts Jan 3 '13 at 20:12
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It turns out a new layout has been rolled out across the network, and my browser was still caching the old CSS.
A force-refresh resolved the issue (thanks Tim).
Update: As Nick points out this was caused by a stale CDN data.