It always bugged me that the liar participates in explaining the rules. That, and they never explaing that saying “It’s a piece of cake” is the reason she falls down to those creepy grabby hands.

As a kid, I always assumed she went through the wrong door despite solving the riddle because the rules of the game were explained by a liar, and that they were both liars.

Since everyone in the comments seems to know, what’s this a reference to?

Be careful, he lies!