Metal Gear Solid's long history of convoluted plots has naturally seen long-standing characters appear over numerous games, but MGS 3's quirky time-bending game mechanics allowed you to mess with this continuity, albeit in non-canonical ways. There's a moment in Snake Eater where you can knock out series' villain Ocelot in a cut-scene, and say all your flow-charts for the series' lore were in order you might be tempted to off him right there - saving yourself the trouble of stopping his crazy streak across every other game. Of course you can't actually kill Ocelot and negate the other titles, but try and you'll be treated to a twist on Metal Gear Solid's iconic Game Over screen. In a goofy alteration on the regular screen, characters will scold you for causing a time paradox in the series and you'll have to load up your last save. It's probably for the best though, the Metal Gear Solid series is confusing and silly enough without dabbling in time travel. https://www.youtube.com/watch?v=me-WHitlw8A
