It looks like the link here is trying to deep-link the PDF, which will work if you've already visited it "properly", but not if you haven't (haven't dug into what precisely it's checking, maybe a cookie, or your IP). You have to click through from here: http://dl.acm.org/citation.cfm?id=2967948

Interesting, most people would say "the milk went sour" or "the milk soured," and not that it went "stale." "Stale" as a word just sounds weird in the context of a drink.