Exactly. Prototyping reveals a lot of engineering constraints that you cannot see just by designing at a high-level. I've been bitten too many times by thinking

Exactly. Prototyping reveals a lot of engineering constraints that you cannot see just by designing at a high-level. I've been bitten too many times by thinking something is simple because I designed things at a high level (without prototyping) only to find out there are hard constraints as soon as I start writing some lines of code. I don't know if this is a common technique but I actually like to do a "big bang" type of prototyping where I build out the major pieces all at once to see what the general look and feel of the system is. From there I can make a way more informed decision on what I need to do versus something superficially high level. For me, the most important thing from early designs is answering the question "what are the unknowns and what are the risks of this project?" and figuring out the right path to remove the unknowns and reduce the risks.