I think the cake was not a lie for these reasons:

1) In Portal, GLaDOS says "you will be baked and then there will be cake" you would be the cake had you been baked
2) when fighting GLaDOS the blue sphere mumbles a cake recipie
3) there is a cake at the end of the game in some storage room that houses more GLaDOS personalities
4) after you beat the game in the menu there shows another cake

the writing on the wall saying there was no cake was just because that test subject didn't get far enough to get the cake.

The cake was a lie.

I beat the game and got no cake, hence it was a lie.

End of story.

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