I've actually had a commercial beer that was bad. A friend brought over a sixpack, he had one of them previously and said it was horrible, I opened one and tried it, it tasted fine, drank it all. Another friend opened another one, he's like "Man, this tastes weird" I took a sip and the first sip tasted like water, took another sip and it tasted HORRIBLE. Harsh and bitter, almost had a perfume smell too it, really thin like water and horrible tasting. No idea how it was bad like that, I guess bacteria?