Marathon has this thing where you're meant to save human staff from alien invaders, but their AI makes them wander carelessly, so they end up walking in between you and the aliens during a shootout, or even walking towards the aliens (who then proceed to kill them).
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Also, I couldn't find a pause key in this game. The Esc key makes you quit to the main menu, instead (but it thankfully asks for confirmation if you haven't saved recently)
I like how you can see parts of the map by interacting with the terminals at the start of each level, though I would've liked to be able to see the waypoints in my own map!