there's nothing more glorious than fan-fictionists and the works they share, but they sometimes deviate from canon in the tiniest ways. the five most distressing mix-ups for me are:

• pepper-up potion vs. invigoration draught
  • a pepper-up potion cures a cold while an invigoration draught gives a surge of energy. if you've stayed up all night shagging, you probably don't need a pepper-up potion to make it through first period, but you might want to take an invigoration draught.
• parseltounge vs. parselmouth
  • if you are a parselmouth, then you can speak parseltounge.
• occlumency vs. legilimency
  • an occlumens employs occlumency to occlude their mind (shield and protect it). it is an introverted and primarily defensive technique. its opposite is legilimency, an offensive technique used to force yourself into the mind of others to some end (usually to "read" their mind). one cannot use the the spell "legilimens" to defend ones mind nor can they occlude themselves into someone else's mind (though special cases where two minds are "linked" could prove a theoretical exception).
• ages
  • harry is the youngest person we explicitly know of in his year, while hermione is the oldest. malfoy is thus two months older than harry and nine months younger than hermione. the marauders are also twenty years older than harry, circa 1960 to harry's 1980. (as such, if say prof. lupin is having creepy thoughts about young ms. granger, she'd be fourteen in third year and nearly seventeen the summer after the d.o.m. fiasco.)
• peeves
  • peeves is a poltergeist as old as hogwarts itself. he wasn't a victim of voldemort; he "came with the building" in the year 993. while a menace to students, he isn't all bad. he aids the weasley twins, gives dolores hell, and fights to defend the castle in the battle of hogwarts. he fears the bloody baron and being banished.