In Akumajou Dracula X: Rondo for Blood for the PC Engine, there's a hidden staircase right before you fight the final boss. Once you walk up the steps to Dracula's chamber, you just have to jump around, hold up, and eventually you'll latch onto the invisible steps, carrying you to a secret room filled with cash. I've known about this for years and years, but what I didn't know was that this is actually a reference to a glitch from the Famicom Disk System version of Akumajou Dracula, the first Castlevania game.
I'm a little unclear on how to trigger it - this site gives some details, and this Youtube video shows it in action. It seems like you have to walk up and down until you "catch" the stairs a few pixels off, then walk upwards, past the actual steps, and continue on into oblivion. Obviously, there's nothing actually up in the sky other than glitched tiles and your eventual death, but with its allusion in Dracula X remarkably self conscious, and another bit of random depth in a game already overflowing with love and detail. Take note that this only works in the Famicom Disk System version, as well as all of the versions based off it, which include the GBA Mini cart and the Japanese VC release. The American and European carts, as well as the Japanese Famicom cart re-release, fixed this bug.
Symphony of the Night features the staircase in the same area, though its implementation is different. Here, you have to hit the upper wall to reveal a switch, which will in turn unfurl a visible staircase. Different items appear depending on if you're playing as Richter (in the prologue) or as Alucard later on.