I'm quoting cause the man brings up a valid point.
Missing Score and Great Gospel in FFVII, un-clouding the Clouded Mirror in FFX, The Brave Blade in FFV with full stats, Adamant Armor and the Spoon Dagger in FFIV,acquiring Gungnir in FFIII, Getting Gogo in VI, Getting Quistis' ultimate Blue Magic spell in VIII, Acquiring Ultima and Zodiark in FFT, and the weakness of all the bosses in the final dungeon to the Blood Sword in FFII.
Seriously, almost every game in the series has some event or item that is easily missed or completely unknowable without a guide or having a friend with a guide. Also, this whole "SE want your money so they make this stuff so you have to buy the guide" though its true, is also kind of a silly argument considering how prevalent Fan sites and stuff like GameFaqs is in this modern age. Really, of anything, its a ruse to make you actually play the game a second time if you are hoping for a perfect file. Its a cheap way to make you want a second run-through with the exception of New Game +.





