They lied, broke the rules
The two door guards are supposed to be 100%.
One 'always lies', and one 'always tells the truth'.
But they BOTH say the same thing in the beginning, explaining the rules!
They BOTH agree that one always tells the truth and one always lies. If the other always lies, HE CAN NOT AGREE WITH THIS, he would LIE about it!
So he EITHER:
a) doesn't ALWAYS lie
b) the truth-speaking door LIES!
This scene makes no sense when they break their own rules IMMEDIATELY.