In Safari 9.0.1 running on OS X El Capitan 10.11.1, the right-aligned content in the Wikipedia's "Beta Distribution" article is shown off the page limits (see attached screenshot). I couldn't find more articles displaying this behaviour.
The same article is rendered as it should in Firefox 41.0.2.
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Thanks for reporting this. Does that also happen in a private browser window when you are not logged into Wikipedia?