Well, it took me some minutes but after trying several browsers and operating systems I finally got it: with IE 8 (actually 8.0.7600.16385) under Windows 7 (Home Premium 64 bits OR Professional 64 bits) the first post mentioned by Richard is wider than the rest. I just tried the first post, because I have a few browsers, I guess it also happens with the others he mentions. Mistery. This is the image:
This very post, after including the previous picture, suffers from the same problem (text is spilled). Look:
Notice that the previous picture (actually, the previous-previous, that is, the first picture in this post) is actually narrower than the width of other posts (I marked the edge of the other posts with a black arrow).
This doesn't happen in Opera (of course! It is master of width, even in handhelds, so... richard, do something!) nor FireFux, Safari, Chrome, Konqueror, Omniweb, Lunascape, Icab or Aweb, nor in IE8 under XP or Vista (I concede I'm emulating those OSs, but that makes it funnier). Why IE always have to be the dorkish brother?