Already posted it here in the past, answering the same question (and some people seem to like it so here we go). Around 1991 I was writing a DOS game... In a v

Already posted it here in the past, answering the same question (and some people seem to like it so here we go). Around 1991 I was writing a DOS game... In a very rare circumstance the game would crash but it could happen after playing for 15 minutes or more. Sometimes not at all. I couldn't make sense of it. At some point I decided to rewrite my entire game loop to make the game engine fully deterministic: input, time (frame) at which input happened. So that I could then record myself playing the game and replay it fully deterministically. Except this was in 1991 and deterministic game engines did not exist back then. The first time I read about one was on a postmortem about Age of Empire on Gamasutra (IIRC). I even wrote to the article's author telling him: "Oh wow, it's the first time I read about a deterministic game engine. I made one in 1991 but since then had never heard about anybody using one." and he answered, as much excited as I was, saying he didn't know about any game doing that in 1991 either and he liked why I came up with it. Since then it became extremely common: a game like Warcraft III for example, where there can be hundreds of units, has tiny save game files for it only records inputs and the time at which they happened (and btw it of course requires to have a same version of the game engine, or a backward compatible one, to be able to replay the save files). But Age of Empire (1997) is the first one that I remember describing using such an engine. Back to my 1991 DOS game... I rewrote the game engine, wrote a simple recorder recording the inputs, and played and played and played until it crashed. I then replayed the game (seen that now I could): and, sure enough, the game crashed. Huge relief. At that point I knew the bug was dead: I could reproduce it, I knew I'd smash it. Turns out: when the hero had taken an extra allowing it to fire two shot at once and would fire two shots, and the first shot would kill the last thing on the level, then the second shot would keep living it's life during the next level (my logic would keep updating that shot and overwriting memory it wasn't supposed to access), happily corrupting memory until something would make the game crash. It was tricky because it require a special condition. And the only way I found to be able to reproduce the bug was to basically invent the concept of a deterministic game engine. Or at the very least independently discover it. The game was never published but it's how my career started (very long story, for a blog or something). P.S: if anyone know of a game using a deterministic engine from before 1991, I'm all ears (especially if it's an arcade one: that'd really make my day).