Perhaps the button should not become active until at least one question is answered incorrectly?   To avoid accidental restarts. 

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2. News and Updates from Khan Academy
3. Update: Restart the practice activity you’re working on— introducing the “Start over” link!

I'd agree with prioritizing this... it shouldn't be hard.  I suspect a lot of people "try out" a first diagnostic, realize they aren't ready to continue, then come back a year or two later knowing ...

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2. SAT Questions and Feedback
3. Option to Reset SAT Individualized Practice Diagnostic Tests