there are many, many things that regularly confuse the glorious writers of the fandom, and among them are these.

• 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).
• 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.
• harry's age in relation to everyone else
  • 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). it doesn't make much of a difference, but it's still something to think about.
• blaise zambini
  • is it a girl? is it a boy? six damn years to figure out one simple question. spoilers: a "dark skinned" boy, who's african in the movies but who I always thought to be italian due to his name.
• 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 deloris hell, and fights to defend the castle in the battle of hogwarts. he fears the bloody barron and being banished. you won't even know he exists though unless you've read the books.
• religion and muggle influence
  • while there is no God in the series, they celebrate halloween and christmas and easter, not yule and samhain and the equinoxes. this leads to questions about purebloods celebrating these holidays and whether purebloods are right that "filthy mudbloods" are ruining their way of life.
• S.P.E.W.
  • the society for the promotion of elfish welfare (which was first named "S.O.A.F.M.C.C.C.L.S." or "stop the outrageous abuse of our fellow magical creatures and campaign for a change in their legal status") is a plot device to show the glaring differences between muggleborns and magically raised wizards. elves (almost unanimously) don't want to be freed, but hermione, being muggleborn, refuses to accept this fact. it can be seen as a parallel to dumbledore's "for the greater good" mindset in which he manipulates the lives of everyone around him to shape the world as he sees fit, deciding what is right for everybody regardless of whether or not it harms or benefits them. S.P.E.W. is not actually about the elves.