My pet grammar peeve: people who pedantically insist on correcting grammar when the correct use is currently ambiguous.

"Their vs. they're vs. there" - this type of correction is fine. The distinction between these words is for the sake of clarity and understanding.

"They is not singular. Trust me on this one." - okay, now in this case, there is historical precedent on one side, and wide current usage on the other. The historical precedents are based on the idea "when in doubt, use he", like in french, which english draws a fair bit of heritage from. The current usage is based on "hey.. why is male the damn default?", a sentiment I can sympathize with. There is no gender-neutral singular pronoun in english, none of the proposed new ones have caught on, and so they has been pressed in to service. Whether you like it or not, english is a living language.

"Dude, it's supposedly, not supposably" - this is cool too. English is alive, but it's based on consensus. If you make up new words, hey, maybe it'll one day become canon, but in the meantime you can expect to be snickered at if there's already a serviceable word in the lexicon in wide use.

"Actually, the correct plural of this obscure word is grandelfrözii, not grandlefrözes." - English is full of weird exceptions to general rules, but the natural tendency has been over the last hundreds of years is for words that are NOT in frequent use to align themselves more with the general rules. Very common words will keep their exception status - like the plural of child is the weird children, because everyone uses the word children constantly, so it's not likely to change much - but less common words will start to use common suffixes, because no one actually uses the weird exception spelling enough for everyone to know about it. Case in point: octopus. In and prior to the fifties, the plural was octopod. Have you ever heard someone say octopod? Of course not. Now, today some people will insist that the plural is octopii. That was pretty popular in the seventies, and it's an acceptable use because lots of people use it, but it doesn't have that spelling because that's the god-given official spelling, so get off your high horse and don't correct me when I say octopuses. Not enough people hear octopii these days either, because there aren't any more octopuses now than there were in the fifties, so once again the word is heading in the direction of a more common suffix.

Does everyone understand? Good children, I'll give you a children biscuit.