This. I have a very short list of architectural must-haves that I use when evaluating junior dev's code at the start of a project, and not having robust error handling that preserves submitted form information is right up near the top.
It's an annoying design mistake with two input elements, unworkable with five, and just plain cruel beyond that.
"Hey, here is your test and here are the answers. You've got one hour" :)
Just handing someone an expanded version of "implement good error handling" is not likely to result in a good product. Every project has nuance and subtle trade-offs -- the code review is an opportunity to teach juniors how to map abstract design principles onto concrete implementations without falling back to "cookbook / cargo cult" programming.
I want them to take ownership of their own code, and advance down the path of craftsmanship because I believe this results in long-term value -- both to them personally and to the project at hand.