Yes it is... But it depends on what you define as legitimate :)
Copied from this Youtube video:
Ok, I have no idea that this is the intended way to get this achievement,
but it works.
Here is how:
1) Close the game!
2) Edit your config file, it is located in your Steam folder. For example: D:\Steam\SteamApps\common\The Stanley Parable\thestanleyparable\cfg\config.cfg just open it with a text editor
3) Add this line to the file (in a new row): bind "x" "~;_u"
4) Save the file then right-click it and chose properties.
5) Check the attribute "Read-only" and save this change. This will make sure that the game won't overwrite your new key bind
6) Open the game and start a new game
7) Hit the "x" key
8) Wait a few seconds
9) Achievement Unlocked!
Thanks to the Steam user AoM, Phone_Answerer and nairolf for this method :)
If you try to enter
_u into the console, you'll get a strange error:
Hey don't you even try
Also, if you try to enter
~;_u in the console, you'll notice that the
~ is a forbidden char in the console. That is the reason why you need to make a key bind which executes this command.
Someone on the Steam forums also decompiled the game and found this code:
if ( !sub_104B22A0(v26, "map1", 4)
&& RandomInt(1, 15) == 8
&& *LODWORD(dword_107E1B28) > (float)(*(float *)&dword_107E1898 + 20.0) )
v27 = *(_DWORD *)(sub_1027BBA0() + 28);
v28 = *(_DWORD *)dword_107E1B30;
v41 = (int)"_u\n";
v40 = v27;
v39 = dword_107E1B30;
(*(void (__cdecl **)(int, int, char))(v28 + 156))(dword_107E1B30, v27, "_u\n");
So as you can see there is some kind of randomness involved
&& (RandomInt(1, 15) == 8)
The developer also commented on this method:
I don't really approve of reverse engineering the game and then divulging the way :/
He also posted this:
The fun part of the achievement was having people try to work out connections really. I updated the code of the achievement every patch to account for whatever you guys discovered. Oh well, no fun allowed for developers I guess.
It's no fun if everyone knows it's almost random!
In one patch I made it so quicksave & quickload would grant it, and in the next patch I made it so it was impossible to get through quickload. It was hilarious to watch people like the original poster raging :P
You can find these comments (and more) here: http://steamcommunity.com/app/221910/discussions/0/792924412213692581/#p54
I also wrote a blog article about this. It's a bit more detailed for anyone who's interested: http://www.themutegamer.com/guide/the-stanley-parable/unachievable-achievement/