there should be a seven step program for this
The biggest reason people do it is because there is a certain percentage of twerps on these boards who see typographical errors as legitimate ad hominem attacks. More than once I have seen those responding completely ignore the content of a post and go on at length about the miss use of a word (their for they’re, etc) They usually quote the error and then say something like, “Well it looks like we can all see just what level of intelligence we’re working with here.”
The only way to avert this sort of hijack is to beat the twerps to the punch and acknowledge the error first.
I was going to say almost EXACTLY what Degrance said. I will also say that I notice this more in the Pit and in Great Debates, where things can get pretty heated, but I’ve seen enough of it elsewhere to know there are plenty of posters who jump on typos as “evidence” that the person who committed the typo is an all-around idiot whose opinion can therefore be dismissed completely.
Sometimes, it is just done in fun, particularly when the person who commits the typo has done so in correcting someone else’s typo 
I read & reread my posts usually & still some get through. What’s funny is that they often make sense too. I might use ‘I went to the beach two’ when I meant ‘I went to the beach too’…
I was reading A MANUAL OF EMAIL STYLE, & he said those who correct other people’s grammer/spelling are called ‘net weinies’.
i’m just happy not to see “your” being used as “you’re”…
On a related note, what’s with people posting to tell us that they just double posted? Hello! We know double posts happen, we’re not going to assume that you did it on purpose just to slow the hamsters down. So you don’t have to deliberately slow them down some more just to appologise for slowing them down.
Catholic school.
Peace,
mangeorge
I hate typo correction posts. I understand the urge, as it is frustrating to realize you made a stupid spelling/typing error in your otherwise perfectly worded post, and now people are going to assume you’re a moron, but the urge must be resisted. RESIST YOUR EVIL URGES!
I like the ever poplar: “Preview is my friend.”
[lame joke]
Oops…popular. Should have previewed, I guess.
[/lame joke]