When I quote people who make typos, I generally have five options:
1) Reproduce what they said and don't comment on their mistake.
The positive side of this approach is that I don't get dragged into silly wars over spelling and grammar.
The negative side of this approach is that it looks as though I may have introduced an error into the comment I am quoting.
2) Reproduce generally what they said but fix their mistakes without telling the reader.
The positive side of this approach is that the end product is clean.
The negative side of this approach is that, technically, that's not what they said, and if I make a mistake in correcting, it may appear I am trying to misrepresent.
3) Reproduce generally what they said but fix their mistakes, telling the reader, but not telling the reader where the mistakes were.
This has roughly the same positives as (2), and roughly the same negatives, although if it is clear that the comment has been altered, there should be less concern that any changes were supposed to secret.
4) Reproduce generally what they said but fix their mistakes, telling the reader where the mistakes were.
This is almost as clean as (2) or (3), and removes the concerns about secret changes.
5) Reproduce what they said, without fixing their mistakes, but comment on the mistakes.
This has the advantage of avoiding introducing my own interpretation of the spelling/grammatical issues in the item I'm quoting.
This has the disadvantage of not being a very clean product.
An additional downside to (3)-(5) is that they can make me look like a linguistic snob, which is (of course) not the point.
In general, I favor (5) in contexts where it is hard for the reader to go back and check the original and (4) in contexts like comment boxes, where the reader can easily go back and check the original. I know I've used (3) a few times, and I have used (2) once or twice. I generally try to avoid (1), but I'm sure I must have used it (either intentionally or unintentionally) at least once when speed was more important to me than quality.
Obviously, there are a few people out there who post supercilious screeds. If someone argues, "Don't listen to TurretinFan because he's an ignorent fool," it's a pretty safe bet I'm going to throw a "[sic]" in right behind "ignorent" - if only to make a minor rhetorical point (not to score points, but to make a point).