Each step they take is an opportunity to learn. It's groundbreaking stuff they're doing. Years of testing certainly does not mean it is as good as it would ever get, it only means it was well tested before they put it on top of a rocket and blasted it off to another planet on the schedule and budget they had. Thankfully they had the foresight to engineer a system that is so robust as to be able to diagnose a sensor issue from 300 million miles away, have the ability to upload a software patch workaround, and be able to resume their tests all within a few short days.