Hidden pint-glass QR code is only visible when filled with Guinness:
A clever bit of advertising gimmickry from Guinness: these pint glasses bear QR codes than can't be read when the glass is empty, nor when it is filled with amber-colored beers. But when filled with black, murky Guinness, the revealed QR code can finally be scanned: "it tweets about your pint, updates your facebook status, checks you in via 4 square, downloads coupons and promotions, invites your friends to join, and even launches exclusive Guiness content."
Yeah, so the last part is a bit of a nightmare.
Guinness QR Cup