This in my theory on the cake. I'm not saying I'm right nor am I saying I'm wrong. I know that the pastry at the end of the game looks like a cake very much, but I like to keep my horizons broad. So let's just say, for the sake of the story, that ending scene in the game never happened where you see a cake.
My Pie theory with the cake actually works. Cause with GLaDOS, she is always saying "you will get cake" or something along those lines. And then the walls are always saying in blood "the cake is a lie".
So here's how "the cake is a pie" works. What if GLaDOS just thought the cake is indeed cake, but is unknowing to her, a pie? Only the ones who wrote on the wall knew this 'cake' is a pie. So when they are writing on the walls "the cake is a lie" they are actually saying "the cake is not a cake but a pie." Thus coming out as "the cake is a lie", which is technically true. It is not cake, therefore a lie in what GLaDOS says about it being a cake.
So in the end, there was a cake that was in reality a pie. GLaDOS just failed to ever notice this cake/pie difference before her murder.
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