The original Castlevania game had lots of hidden bags of treasure that would only appear if you squatted down on a certain block or stood still in a certain place for long enough. However, there's one treasure sack that can't be collected by any means. Why's it there? No idea.

First, make your way to what the game calls 'Stage 9', but what is actually the end boss of the third area. Find this block:

Now, just press Left for a few seconds and viola! A money bag that pops up in a completely inaccessible location.

Ultimately, this is kind of pointless, but you might be able to use it to win a bet or something.