I thought this was going to be a post about videos games.
Though to speak about building video games, build with multiplayer in mind first because it will force you to make your game deterministic which will save you a huge re-write.
When I was building Swap-N-Pop (Tetris attack clone) so many other people tried making clones as well and they stopped when the found out that their game was non-deterministic and they'd have to overhaul their game to achieve their goals (community multiplayer game).
Also, write test code on day one this will help you hold less state making debugging easier as well.
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