I do not know if this is a current problem, but at least some years ago, the CVS Extra Bucks coupons were generated via a thermal printer. I inadvertently discovered that if I stored them in a shirt pocket on a hot day they could become virtually unreadable, and the store (understandably) refused to accept them.

A friend who worked briefly at CVS told me that the stores were planning to switch to inkjet printers, though I can't confirm whether that has happened. For all its faults, I prefer the Walgreen's system where I do not need to carry around miscellaneous pieces of paper.