What are the most common mistakes in system design interviews?

I've been conducting a lot of system design mocks recently, both internally and with external folks, and I'm seeing some recurring patterns in candidate struggles. It's interesting how often the same pitfalls appear across different experience levels. One common issue is diving too deep into a specific component too early without establishing the overall system context. Another is not asking enough clarifying questions about scope or non-functional requirements upfront, which leads to designs that miss key constraints. Over-engineering for features that aren't in scope is also pretty frequent. What are some of the other major mistakes you've observed? How do you gently guide candidates away from these issues without giving away the solution?