What happens when girl scout cookies expire?

Expiration dates, on girl scout cookies like all foods, is a guideline after which it is expected the product will lose flavor, "freshness", and/or nutritional value. Some foods, especially refrigerated ones like milk, may even become contaminated by microbes or bacteria that can be harmful to your health. However, there is no sudden "switch" that occurs on the expiration date that causes the food to suddenly become inedible. Rather, it is expected that around that time the food's quality will begin to deteriorate. Expiration dates often void return policies as well.

Specifically for girl scout cookies, after the expiration date they will likely become stale and less tasty. If it is not too long after the date, you can probably still eat them. However if it has been, say, years, you should probably just throw them away.