I'm playing Super Castlevania IV right now and there are several areas where, if you aren't paying 100% attention, you will fall into some trap or be completely blindsided by an enemy. Also in the game you can use your whip to swing onto other platforms. Normally the swing will take you straight to the platform so you can just let go and drop down. However, in stage 3 you can actually swing PAST the platform, so if you release the way you normally did, you will fall to your death. There's a lot of these subtle changes that can completely screw you. In stage 6 one dog takes one hit to kill, but the very next dog takes two hits. So you will probably just hit it once and keep going because you assume it's dead, but no.
Also stairs. You can climb up stairs, but if an enemy knocks you back into them, you just fall through them and to your death.