I did get killed by that spider on the first screen a few times. And the crystal mountains could often be a pain. However, that fragile cloud bridge with the skeleton at the bottom caused me some big headaches. No matter how careful I could be at times, there were many occasions where it caught me out. Perhaps I was being too careful. In fact, some of the time I'd be crossing that bridge, expecting it to break, so I guess that was a self-fulfilling prophecy. Whatever the reasons, I lost more lives on that bridge than anywhere else in the game.
What about the rest of you?