I've only sent back one beer in my life; it was after just two sips. The funny thing is that, we were traveling and had stopped at a small cafe to eat. I asked the waitress what they had on tap and she listed a beer I'd never heard of before, so I asked who had brewed it. Rather than tell me the name of the brewery she just told me the brewers name; it was an old friend that I hadn't seen in a long time. I think the beer was an american amber; it was completely, horribly, sour. Everyone else in the bar was drinking the amber ale and raving about how great the new brewery was (their first brewery). I cracked up and told her that just because the brewer was a friend I would not be finishing the beer and I didn't care what everyone else in the bar thought. To be fair, I know that since then they've cleaned things up and I'm betting their american amber is good.
There was a Ft Collins brewery that has since gone out of business, I recall hating every single one of their brews.