There are three I can think of.
Fora. I know it is correct, as is forums, but this for some reason wrecks my head when I read it.
Faux News. I read this as “Foh News” and it irritates the bejakers out of me. I can’t explain why.
Shrub for GW Bush.
Sorry, I really don’t know why these irritate me but they effing do!
I realize you’re not from the US, but these aren’t SDMBisms; they’re Americanisms. Faux News is the liberal interpretation of Fox News. George W. Bush has been called “Shrub” since he was the governor of Texas. As for “fora”/“forums”, that’ll happen anytime you’ve got an intelligent group of people who know it’s “fora”.
Don’t let it bug you. Really.
Robin
I hate the use of, “Sorry, but …” when prefacing a correction, or more often just a different opinion or interpretation. Is it supposed to sound courteous? It just comes across as completely condescending.
I also hate the horribly twee “methinks”. Who talks like that?
Come to think of it, who says twee? 
Umm, cite?
d&r
It irritates the hell out of me, for reasons I can’t entirely explain. Mostly, I guess, if you’re going to make a stupid joke, stick around and take your lumps for it, if lumps there are to be had. Usually the joke is so stupid that it doesn’t deserve anything more than a rolling of the eyes, if that – so there’s no fucking need for you to either duck or run.
I don’t think I’ve seen any of those three on SDMB at any regularity. And anyway, they are hardly endemic to here.
Spelling/grammar/punctuation nazis.
This.
Faux is pronounced “Foh”, so I’m not sure why you would be irked by reading something the way it is meant to be read?
Fora. This pretentious (correct, but pretentious) usage cropped up as soon as there was an Internet with college graduates on it.
Faux news. Never heard this on SDMB. IRL I do hear it. Pronounced as you said. It’s a pun, see?
In our household GW Bush is Shrub Jr. His daddy was Shrub.
Now then, for some actual SDMBisms that irk me:
The whole “Rio by Duran Duran” syndrome, that is, where two people give an answer, say in GQ, and then a whole bunch of other people come in saying “Oh, just a WAG but…” and then repeat the obvious, already given, other answer. There’s a way to do this that doesn’t look like you think you’re being clever. That way is rarely taken.
The fact that there has to be a #3 and it has to say “Hi Opal.” There does not have to be a number three. You can do a numbered list with only two numbers. You can’t do a numbered list with one number (and even a bulleted list would look stupid with one number) so Opal was just wrong, in the first place, and it’s gotten very old, in the second place. And there is no third place.
Repeated pittings of the same person for the same reason. Yeah, _____ is a _____. Deal with it. One pitting per person per issue so we don’t get “Oh, is it that time again” or “Gee didn’t we just do this two years ago?”
Saying, “Um, cite?” when you don’t actually want a cite but just think it’s the thing to say.
Saying (typing) “Um” in almost any context.
That is all. For now.
I also hate this. I actually don’t have very strong feelings about words (you’ll never catch me posting to one of those “words you hate/love” threads in IMHO, cause mostly I don’t give a shit), but “methinks” causes me to dislike the poster.
Of course, I only remember it for five minutes, so it’s not a big deal. But it is extremely twee. Just like SkipMagic.
There is no goat. There never has been, and even if there had been, bestiality is gross.
Never was a big fan of all the “initiation” shenanigans. Goats, squid… WTF?
Yeah, the initiation crap may be the singularly most stupid practice on this board, and considering the inanity of most of the in-jokes here, that’s saying a lot.
“Meh” really gets me. You don’t say this word in real life, well, I don’t, nor have I ever heard anyone else. It looks stupid to me for some reason, please stop. What the fuck is “meh”? I mean, I know what it implies, but it’s not a real…thing…
Um, cite?
An SDMBism I’ve noticed recently, although not particularly irksome, is quoting someone else post but editing the text in a humorous manner, then adding “Fixed that for you”.
Fix’d
There is absolutely no reason why you can’t say “fixed”; there is no need for an apostrophe. I can see why some people dislike “ninjaed” with the whole double vowel, but there is nothing wrong with the word fixed.
Valete,
Vox Imperatoris
(Now someone’s going to criticize signoffs.:p)
I kind of like “meh”. The first time I heard it, I knew exactly what it meant: it’s somehow the perfect expression of apathy.
Valete,
Vox Imperatoris
“nuff said” punches my buttons. I have no idea why I react so strongly, but it riles me up every time.
The best I can describe my reaction is that this is a message board … there could be upwards of 100 posts in a thread, obviously it wasn’t “nuff said” because people keep on saying things. And many times it’s in a thread where including an explanation is the point. If someone is asking for TV recommendations, and someone comes in with “Buffy … 'nuff said!” it’s maddening that the poster couldn’t be bothered to add why he likes that show, or why he thinks it would be a good recommendation for the OP.
Oh yeah, and the idea that “nuff” is a good way of saying “enough” is annoying on its own.
This. 
Seriously, I hate that. I wish people would stop. It’s almost as bad as +1.
qft.
(Why won’t it let me capitalize QFT at the beginning of a post? Bug in the board code?)
Valete,
Vox Imperatoris