If you've been drinking beer long enough you've encountered skunked beer at some point. So how does a beer get skunked? First thing you need to know is that the professionals don't call it skunked, they call it lightstruck.That's important to know because it ends up that the popular wisdom that letting a beer get hot will cause it to get skunked, er, lightstruck is incorrect:

What those researchers found is that there are two distinct pathways to getting skunky-smelling compounds in your beer. The two main actors is this tale of woe: hop alpha acids and light. Not heat. Not oxygen. Light…

All beers that have been bittered with hops can suffer skunking: As an experiment, get a draft beer poured into a clear glass and then let it sit in the sun for 10 minutes or so. Compare that beer to one fresh from the tap. You should definitely detect some skunk in the lightstruck beer. With clear, green or blue bottles, the glass doesn't filter out the ultraviolet and blue wavelengths that start the skunking reaction. Brown bottles are much better at keeping those wavelengths out of your beer…

In the end, if you want to avoid the skunk entirely, just buy a beer that has been packaged in a keg, cask or can. Those beers can (and do) develop bad flavors, but you'll never get one that has been skunked.

Duly noted.