Leadified on 11 February 2015
Haven't played too far in the Dark World levels but...
6-5: Omega in Super Meat Boy was difficult. Long level with some tricky jumps.
Combined with the fact that Super Meat Boy on PC is really finicky with hardware/software settings, it's possible to make the level impossible until you fix the issue.
The last few levels in Ninja Gaiden on NES are just insane.
Edit: I just remembered since Ikaruga came back to mind.
https://www.youtube.com/watch?v=iPx-oznzdTs All I can say is, thank goodness for unlimited credits.
