Visit http://en.m.wikipedia.beta.wmflabs.org/wiki/Barack_Obama?mobileaction=stable
Uncaught TypeError: Cannot read property 'length' of null
initButton @ init.js?054e7:170(anonymous function) @ init.js?054e7:271fire @ load.php?
I'd say this is UBN since it causes a JavaScript error for all users in stable which can break other functionality (for example watch star doesn't work for me).
Should fix before gets on the train!