I'm pretty sure that I've had prescriptions where the pharmacy charged more than the insurance company would allow and I got stuck with the overage. It's pretty rare, though. More common, in my experience, is where the cost is actually below my plan's copay and I get it cheaper.

But you're right, I don't do pharmacy comparison shopping. At the same time, I don't really have a good way to. It's not like I ever really see the final cost at all, from either the pharmacy or the insurance company. I think that if I ask for a report from the pharmacy it will show me the actual final cost, but I'm not even sure of that.