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I've just played through Stanley Parable for a few hours and have got all of the different endings (including all variations of the easter eggs), besides the "disco" one.
After quitting the game, I noticed that I got the "Unachievable" achievement. Looking at the time, I think that I got it around the time I got the "broken narrative" ending, but I'm not entirely sure.
I've looked up how to get the achievement and most guides involve adding key bindings and doing it through there - I definitely haven't done that. The only things I've done in the console are
sv_cheats 1 and
I am not looking for how to get the achievement, my question is that I have got the achievement and don't know how I did!