If I am not mistaken, on level 16 "You have to open another window", the button to push to open another window is "Walkthrough". And when you push it another window comes up, but it says "Ooops! We can't find that!". So until they fix the link you can't clear that level.
Is it possible to shut off the function that offers you to change your Avatar after every challenge? Starting to get very annoying... Because I will not change
 "This is the Only Level 3"){kind=icon}
 "Super Duck Punch!"){kind=icon}
 "KingsRoad"){kind=icon}
 "Paladog"){kind=icon}