This message (and actually 4 other codes like it) jump out in IE6 only and only during ssl ajax sessions. The only info I've been able to find out is that the packet never left the client's machne, so it is safe to reissue the request. This is important to me, because I am doing this on a transactional system.
I've got a lot of discussion brewing on this one at my forum,
http://www.perkiset.org/forum/ in the Ajax and Javascript sections. If you get more info on this - amd particularly if you can find any more symptoms I'd LOVE to hear them. I've tried complicating things with iFrames and all manner of workarounds, but to get the user a "Locked" https page AND do Ajax you have to go this way.