Not sure if this counts as a feature (probably more like "I missed an entire level"), but I just remembered another good one from Kirby Super Star. In the sub-game "Milky Way Wishes", you can't eat enemies to gain their powers like you normally can; you have to find them within the seven levels you play through, and then you can switch to that power whenever you want (kinda like Mega Man's powers). I managed to get 18 of the abilities you can claim and use over and over. But when I looked at the menu for my powers, something bothered me. Next to the bar that listed controls for the menu (looking up the moveset or selecting it), there was one blue space left that looked like the size of the other images that represented your powers. I was, in fact, missing a power that I could get. I suspected it was the Copy Ability, but I looked everywhere in "Milky Way Wishes", and I couldn't find Copy anywhere.
I didn't know how to get that last 1% until I looked on GameFAQs years later (this was probably around the time the DS version was coming out). As it turns out, the Copy Ability IS in "Milky Way Wishes", but it's found in an EIGHTH level that you can visit. This eighth level (titled "?" when you visit it, but I think the walkthrough called it the "TAC Homeworld") can be accessed on the map screen for "Milky Way Wishes", but
It's one of those things that you can easily overlook at first, but once it's revealed to you, it sticks out like a sore thumb and you feel stupid for not noticing that before.
I didn't know how to get that last 1% until I looked on GameFAQs years later (this was probably around the time the DS version was coming out). As it turns out, the Copy Ability IS in "Milky Way Wishes", but it's found in an EIGHTH level that you can visit. This eighth level (titled "?" when you visit it, but I think the walkthrough called it the "TAC Homeworld") can be accessed on the map screen for "Milky Way Wishes", but
the TAC Homeworld is a tiny green planet on the map. It's a lot smaller than any of the other seven planets you visit, so it's easy to mistake it for just part of the background.
It's one of those things that you can easily overlook at first, but once it's revealed to you, it sticks out like a sore thumb and you feel stupid for not noticing that before.