Items you permanently missed in Mario games (unmarked spoilers)

YoshiFlutterJump

Power Star
MarioWiki
YoshiFlutterJump
You know that terrible feeling you get when you complete an RPG, look at this list (List of missable content), and shout "NOOOOOOOOOOOOOOOOOOOOOOOOO!"? Well, it’s time to share those frustrations on this thread, and warn others not to do what you did. Below I will present my biggest frustrations with Mario games.

New Super Mario Bros. Wii
R.I.P. to 101% completion of this game. Of course I saw the Super Guide block because I’m no expert at this game. My advice: if you see the Super Guide block, immediately reset the console without saving.

EDIT: Like Baby Luigi said below, you can also just never play the game alone.

Mario & Luigi: Paper Jam
Bowser’s Castle cannot be revisited. As such, I cannot go back and get the Bros. Wear. My advice: go back through the area on your own after completing Toadette in Trouble.

Mario & Luigi: Superstar Saga DX
I am at the final boss of this game, but only now do I realize how much I missed. I missed the Bros. Rock badge because I failed to collect the ten Beanstones. I also failed to collect the five Beanlets to get a Golden Mushroom. I missed the Revenge Jeans because Jojora fled in her battle. My advice: get the Beanstones and Beanlets as soon as possible. Defeat Jojora first in her battle, annoying as it may be. And if you’re on the GBA version, make sure you get the Game Boy Horror SP before fighting Piranha Bean.
 
I hate how Golden Mushrooms in Superstar Saga are one-shot items. Because of that, you try saving them for later, for a time when you "need it", but you end up never using it because of its limited quantities even when you actually need it.

For New Super Mario Bros. Wii, you can always play in multiplayer, Super Guide blocks don't spawn at all :^)
 
Sis, I still remember the silky pants in Partners in Time and we both regret selling those at one point.

Never forget the silky pants.
 
I suffered missing the shiny stars in 3D Land. I guess I'll avoid missing it if I ever replay it.
 
Back