The one right after the first lava stage, but I don't think it was badly designed, it's definitely a skill issue from my end :,>
Though maybe a way to make stages easier without nerfing them could be by adding markers, for example add a marker that tells you where to go every once in a while or something (just a sugestion though, I know nothing about 3D level design and I've only played 3D Mario games LOL)
Still a pretty good game, I don't mind the level design I was just kinda bad lol