I gave up at "Mark all squares of the 64th floor of the Empire State Building"[1]. I had even spent an hour on the chess challenge, but that one looked tasteless if I wasn't missing a trick. I thought that I was supposed to try all the floors close to 64 (considering the topmost one below the tower was 86). I really didn't have the patience for that, especially the possibility of missing to mark an overflowing pixel line, etc.
Also, I hit a bug at the AI challenge which prevented me to pass it. So I had to spent at least 5-6 more tries to pass it.[2]
Fun, but wouldn't go near it again :)