Maybe not a problem with Nabble — I saw this on only one computer — but for the record …
When I read (expanded) a message, then 'rolled up', the read message appeared to be unread (bold):
The problem appeared with various posts, viewed in
Safari 4.0.5 (6531.22.7) in Mac OS X 10.6.3 and in
WebKit nightly Safari 4.0.5 (6531.22.7, r58209) on the same computer. I could not reproduce the problem in Safari 4.0.5 on a different computer.
I can't say exactly when I first noticed the problem, but AFAIR I didn't see it before the DN changes.
Resolution/workaround in this case:
1. remove cookies relating to nabble.com
2. quit Safari
3. launch Safari
4. log in to Nabble.