Firstly, apologies for cross-posting - this should probably move over to zope-dev henceforth. Secondly, thanks for the replies. In my small sample, it appears that 3 out of 7 IE browsers got the bug - no other flavours had a problem at all. They were mostly versions 5 and 5.5. If there was any pattern, it's possible that 56-bit versions were the baddies, but that's not clear yet. As Martijn correctly pointed out, there seems to be a Content-length problem going on here. I did a lot more tests on two different 2.2 servers, and have come closer to isolating the bug. Contrary to what I said, it *is* triggered by bad html: the </head> tag was missing. In documents of a certain length (at least 2000 bytes), which contain the <base href> tag, but don't have a </head> tag, the Content-length is consistently reported as 14 bytes too short. This is regardless of the actual length of the document. Why this should have a knockon effect on a small subset of MS browsers, I don't know. Especially confusingly random is the requirement for a certain amount of data to break these browsers. Even more mysteriously, if I replicate the data which zope is sending to the browser using netcat (bad content-length, same content), IE doesn't complain. Things like Keep-alive perhaps spring to mind? Anyway, I have fixed the problem in terms of my users by adding the </head> tag, but this looks suspiciously like a ZServer problem as well as an IE rendering problem. I'm off on holiday tomorrow evening and I don't have any more time to investigate. I haven't checked against other versions of Zope, but the one I'm using is only patched in DT_Util.py. This is such a dumb problem it's just *got* to be my fault somewhere down the line :) I'll check against other servers when I get back (mid Jan - long holiday :-) cheers, seb