If you've never been to In-N-Out [I'm truly sorry for you] you might not know that the menu is very limited - single, double, cheese/no cheese.
But there's always been a secret menu - one that doesn't appear anywhere* but most regular patrons know about. Grilled cheese and Animal style are probably the most popular. I don't want to tell you too much. Wouldn't want to ruin your discovery [though here's a pretty extensive list].
*I was a bit disappointed to find a section on In-N-Out's web site that discusses the secret menu. Seems unnecessary and certainly renders the word 'secret' impotent.