And if trying to knock out a whole app like that, you might rediscover why waterfall was abandoned a while ago for many sorts of endeavors along the way... :)

or that waterfall works if you get the app done in less time than the initial waterfall meeting. Because now you have time to iterate after examining the output and testing it out to see if there's shortfalls.

Check out this guy one-shotting a game engine (ala unity-esque) app: https://www.youtube.com/watch?v=R_uf5OfMGio