Out of curiosity, were you using Internet Explorer and was IE open (esp on this site) for more than a few hours since you last closed all the IE windows (so that it could restart).
IE seems to become very unstable with the new site if you leave the browser open and running for more than a few hours at a time. This instability always seems to become most apparent during the composition and submission of replies.
Some of this behavior is almost certainly related to bugs in IE that Microsoft has yet to repair (it can take MS as long as 5+ years to fix even well-known bugs), but some may also be bugs within vBulletin. Hopefully the next update to vBulletin might improve things, but we won't know until afterwards. In my own experiences, IE9 has been far less stable and reliable in general, not just with this site, than its predecessor (especially on slower machines).