As I mentioned a while back, I use a ton of sunscreen. I am also a bit scatterbrained and tend to lose things like sunscreen, so over the years, I have bought and squirreled away innumerable bottles of both "regular" and "face" sunscreen (critical to prevent serious pain from sweat + sunscreen running into my eyes).
I am also cavalier about expiration dates.
This isn't usually a problem, but I recently found an old bottle of sunscreen and in a pinch, applied it to my face. Two days later, I had a horrifying rash + acne breakout, and checked out the offending sunscreen.
It expired in 2009.
In hindsight, the fact that the sunscreen was not white (or even sort of yellow) but a light brown color should have tipped me off that perhaps the stuff was not trustworthy. I think I've used it relatively recently (like, a year ago) with no ill effects, but I may have restricted it just to non-sensitive areas like my arms.
So I actually went through and pitched everything that expired more than 3 years ago. In the future, I'll try to keep my purchases of new sunscreen to a minimum, and keep my sunscreen storage places to a few safe locations (i.e. not in the car, where they alternately cook and freeze). I would like to avoid looking like a dermatology case study.