The loading of the wrong reply issue should be fixed now.
It's not the whole post, though - just the avatar usually, and once or twice it's been the stars from under the poster's name.
I haven't been able to replicate this, but it's possible that the fix I just put through for the other problem has also fixed this. If it hasn't been fixed yet, does it happen in particular threads? Can you link me to one?
An overall clarification: the tooltips code has always been a little shaky, and I finally gave them [link=/forum.html?forum=3&mid=1198279925234113786&p=2#91]a complete rewrite[/link]. Not just for forums, but for all our database tooltips. Tooltips should load faster, attach to the mouse better and more smoothly follow it, better handle re-positioning when necessary, attach to the corner of icons when hovering over an icon link, and a bunch of new features for external features are now possible, like dynamically adding icons to links, or changing the link colors to match the quality of an item being linked. Forum tooltips still won't appear on external sites, but they benefit from the increased speed and responsiveness, and we're finally able to load tooltips specific to a page of a thread.