I once heard Hofstadter’s Law of computer software engineering:
It will always take longer than you think, even when Hofstadter’s Law is taken into account.
Having just spent the day thinking and talking about fault-tolerant quantum computing, I think there’s a corresponding Hofstadter’s Law of quantum computer hardware engineering:
It will always be more difficult than you think, even when Hofstadter’s Law is taken into account.