That can be due to a few things, but mainly storage. Storing beer at a constant temp is very important. Going from hot to cold is bad for the taste. Also, if you have a beer that has been exposed to sunlight, the UV causes a recation with.. I can't recall what.. to create the same chemical as found in skunks spray. This is refered to as "skunking". Dark bottles help protect against this, but only slightly
Far out! Skunked up beer braaaaaa!