I just upgraded from 3.7.4 to 3.8.4pl2. No problems during upgrade through all versions. The only thing I have noticed is some attachments don't show up. All thumbnails show up properly, but sometimes when you click the attachment to view in the lightbox, nothing shows up in the lightbox. The details show up, just no large image. When I copy/paste the url (like http://domain.com/forums/attachment....9&d=1267597014) that it is trying to retrieve, I get a blank white page. I am using the filesystem to store attachments and haven't changed any of those settings, but double checked to see if they had changed. I also disabled hooks via config, no go. I have checked for suspect file versions and repaired db tables.
Again, this isn't happening for every attached image and all thumbnails do show up. Its just on some images from pre-upgrade. It seems post-upgrade attachments work ok.