Allright, let's play a game, there are two mistakes in this drawing, the first one to tell me where they are and guess them right will get a free request, a sketched one, that is.
A pair of clues:
1: it's not in the artistic execution A: because there's not much art in here B: because it'd be FULL of mistakes and misplaced lines and colors and whatnot.
2: Look at the sky in the game.
EDIT: :iconkalto-mate: guessed both the mistakes right! There's a plastic dome in this level in the original game and the moon is not visible whereas it's always supposed to be. I ignored both these things to make the joke work in the comic (it was way too late for me to change them when I realized what I was doing).