Good design matches user mental models rather than forcing users to understand system internals. When designers fail to provide clear conceptual models, users create wrong ones. If an error is possible, someone will make it—design must minimize error probability, not blame users for mistakes.

Systems must continue working under stress, but excessive buffers create inflexibility. When hierarchies break down, they divide into subsystems. Errors should be easy to detect, have minimal consequences, and whenever possible, be reversible. Good design balances efficiency with redundancy.