I've been having issues with my spam module since upgrading to Drupal 6 a while back. It changed behavior very significantly, and in my opinion for the worse. Part of the problem I've been having with it is that content detected as spam is not always reliably reported as such. Sometimes it just disappears, literally. Submitters have the option to submit feedback on posts falsely classified as spam, and I may see that (if I remember to look in a completely different location than the rest of the posts I review), but even when I do see the feedback, the original post itself seems to be purged from the database.
I've noticed this problem before, but I didn't realize how bad it was. I have over a dozen feedback messages I just noticed for false positives, and I cannot approve the original posts because they no longer exist. Beyond that, there's no telling how many posts without feedback were falsely rejected.
The one good(?) thing is that this only seems to affect anonymous comments (which are heavily moderated anyway). If you want to post any comments to my website or forum, please register an account first - this should make sure your post gets through, and even if it is falsely reported as spam I should at least be able to review and approve it.
To everyone else that's been affected by this - my apologies. I do still have the content of the posts you submitted feedback on (as opposed to the original posts that I can simply approve as "not spam"), so I'll try to manually post them to the appropriate locations as myself and respond where appropriate. Please check back over the next day to see if your post made it.
I'm also going to investigate alternative anti-spam options to try to prevent this issue in the future. I'll write a new post about any changes.
Update: Whew, ended up adding adding quite a few new forum posts and comments. Again, if you've posted a comment that was (falsely) flagged as spam and wondered why it never showed up, please check to see if your post is available now. I apologize once again for the screw up. Hopefully I can find a better spam solution soon.