For the curious, the issue was initially caused by r814049-plugins on December 2nd, and resolved less than two days later (in trunk) as r814959-plugins.

Unfortunately, the first changeset (which should have been harmless -- I'm still not quite sure why it glitched) squeaked out on December 3rd with the point release. We caught and resolved it the next day, but as we were already going into beta for our Jetpack 2.7 release and as it was merely a cosmetic issue (rather than a fatal error bringing down anyone's site or a potential security hole), decided to let the fix ride a week until the public release of Jetpack 2.7.

For anyone eager to get it resolved sooner, rather than modify the file by hand, just download https://plugins.svn.wordpress.org/jetpack/tags/2.7b2/modules/stats.php and replace your ~/wp-content/plugins/jetpack/modules/stats.php file with that version.