Why are some posts being randomly justified with big spaces for no reason at all ?

Probably because next text row contains some long link.

I'm not sure if it works like I think it does, but I think if you mix in (asögjkföbnalöfjgbmeröagjövlkfmeöragjvklröajgmböioergvölaekrjgv&o uml;oaerigjmvoöeiragmlaerivmGUBBAGAAAGU BBAGAAAYOUAREMYHONEYBUNNYsowhydidyoutakemyshoes) a long enough word that doesn't fit the row, the row gets justified. That gibberish there is an empiric test to see if my theory holds. :p

/edit: it did not work! Lexor's link one is probably correct. But interestingly, it added symbols that I didn't type (like ö) and added a space after GUBBAGAAAGU. Most fascinating.

That's kinda annoying, the posts shouldn't be getting justified because of a link...

I guess you don't have font smoothing turned on.

It's called ClearType in Windows. Here's a guide since I don't remember how to turn it on in XP (Vista and 7 has it on by default I believe), you may have to run it in Explorer.

Don't think ClearType is the issue, but the combination of colors used by the forums. It gives the illusion that the small bit you mentioned is missing. If you zoom in enough, you'll see that it is not missing.

EDIT: The same is true for the letter A. I'm guessing either color issues, or the CSS for the green / yellow / blue eggs or whatever is a bit messed up.

The only place I've seen it "missing" is where ClearType has been off, which is why I answered that way.

See my edit.

Another edit: Everything displays fine with Firefox, but I don't think that browser accelerates the text / fonts. IE10 has text / font acceleration enabled (afaik), and the issue is clearly there.

So, either the CSS or HA is the problem. Either way, I'm guessing it can be fixed by moving the titles a pixel or two to the right?

Because when you zoom in, the lines become wider and the absense of ClearType is much less of an issue (it's normally less than a pixel wide and would blend nicely with ClearType, but since ClearType is off, it doesn't blend but cuts off abruptly).

Without font smoothing, adjustments of less than one pixel creates those issues, with font smoothing the colours of the letterforms and background can blend and give the impression of lines being at half (or third, fourth ...)-pixel points).

The bottom left part of A isn't missing a small bit?

That could be it. I'm on Windows 8, and I think I've read somewhere that ClearType isn't implemented yet correctly on it. (yea, I forgot about it before posting my previous posts)

Different browsers often do use different font compositing calculations, yes. Even if they otherwise use the same rendering engine (compare Safari and Chrome for a world of difference - both use the WebKit HTML/CSS rendering engine, but Safari's font rendering is really horrible, especially noticeable when using CSS3 animations and rotating text).