I can reliably reproduce this after several shift+F5s: the old Community Promotion Ads are still showing up (at least on Programmers.SE), from 2014, not the new ones from 2015.
Is this just another case of http://shouldiblamecaching.com? (It would have to be server-side, since even after manually disabling the cache I can reproduce this.) Or is it a bug? Oddly enough, that ad is the only one that I've seen after refreshing at least 25 times, which smells of a strange bug, but it could still just be a caching quirk.