More importantly, though, I feel the need to stop other people making fools of themselves first. I'm not sure about the demographic breakdown of this community, but already I'm suspecting that many if not most people here live in America. More likely than not, most if not all of us here speak good to fluent English, and so should usually have to problem understanding one another.
The fact of the matter is, though, that English is different wherever it is spoken. A doughnut may be called a cymbal, an olycrook, a cruller, a friedcake, a belly sinker, a dunker, a fatcake, a fasnacht, or a bismark, depending what part of America you're in. If you've ever heard someone use "but" or "only" as an intensifier ("It was she only who cooked this rice") they may have been speaking Indian English (in which case my example sentence is completely acceptable). There are phrases and idioms in certain areas of the world that don't exist in other places. English also borrows many words from other languages, so while you may talk about "adobe" and "tortillas" in the American Southwest, someone from Edinburgh may just as well have no idea what you're talking about.
There are more differences in English in different places than just the accents.
"Fug", which many people are believing to be a typo, is informal British for smelly atmosphere (or fog, whatever). When Ron talks about taking a shufti, he means to take a quick look around - the word originated as slang in the British military and it crept into widespread use sometime after World War II, if I'm not mistaken.
If anyone else has a word or grammatical phrase they think may be a typo, I propose you post it here before you blab to all your friends that JKR doesn't know how to use spell-check. I highly doubt there are any true typos in Half-Blood Prince; such a highly anticipated book would have been gone over with a fine-tooth comb by various editors as well as the author herself. If you see an unfamiliar word, I will go out of my way to find you the etymology and prove that it is a real word, just as I will do a headstand if that's what it takes to defend any grammatical constructs in the book.
I almost wish I were a prefect or other such officer, because then I might make this a challenge and give out points for it. But hey, I certainly wouldn't mind if any current officers did decide to make some sort of game out of this (just give me credit, 'kay? I'm new and I'm trying to get well-known).
I - well, I guess that's all I had to say.