Guinness? Don't get me started! (Too late...).
Not so long ago (and I'm not that old!) culturing the yeast out of the bottom of a bottle of Guinness was one of the few ways of getting decent yeast for your homebrew.
And the "real ale" revolution (and CAMRA) came by, and lots of breweries changed their methods to suit (there was only 4 pubs in England brewing their own back then). So what did Guinness do...
Pasteurised their beer and started messing about with Nitrogen conditioning, the start of their slippery slope. I liked Bevvie's description of Guinness; "milk shake". But Guinness have such a history and following few actually come to the conclusion that its now rubbish! Yeah, brew your own, you'll beat Guinness no bother!