Definitely not an error. I believe this is a very typical "Coca-Cola quarter". Drop a perfectly normal quarter into a glass of Coke, and wait for it to stop fizzing, and it will come out looking pretty much the same as this.
The copper core is more vulnerable to acid attack than the cupronickel cladding, which is how the "coin sandwich" effect was created. Acid attacking underneath the plating also explains the bubbles and blisters all over it.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis