It hasn't happened to me (yet!) but I see from the WTA RG thread, that it has happened there.

Activeboard has always made threads (or at least, all the posts on one page on a thread) wider if one post has 'forced' it to do so, usually because too large a picture has been posted.

If I remember right, the post that set it off this time was the one where korriban posted the link to the Telegraph's post-match article about Laura. I don't know why, because the link itself wasn't too long (at least not on my screen) but the font for the text after the link was unusual, so I thought it might be something to do with that.

I therefore edited the post, copied the post out to a text file, erased it all in the edited post and pasted it back in. That seemed to have solved the problem (in the sense that in the edit box, the text in the original post wasn't wrapping, but after I had taken it out and pasted it back in as plain text, it did wrap, but sadly once I hit send on the edited post, it went back to not wrapping again.

Anyway, it's not an unusual Activeboard glitch (just a rare one) and based on past experience, it should only apply to that page of that thread so isn't really anything to worry about.