Not with Clif Bars, but I found 2 very old Hershey's Tropical Chocolate Bars in the bottom of my old Sac Millet that I hiked with as a teenager. These were Vietnam war era so about 30 years old, sold at places like REI. Dry and a bit crumbly, but delicious in ~2008. They were still wrapped in an old Ziploc so I wasn't too worries about infestations.