I don't believe it's 100% on the dev to find all of the problems. Once you get a look at how the code works, you are less likely to find unhappy cases simply be

I don't believe it's 100% on the dev to find all of the problems. Once you get a look at how the code works, you are less likely to find unhappy cases simply because you know in advance that doing X is stupid so you don't think of it. But of course a user wouldn't know that it's stupid or they are lazy or not as careful as they could be. That's why you need a second person (ideally QA) to look at the result and test it. Cognitive bias 101.