I noticed something odd today that happened twice already, so it looks like something suspicious. I open my forum and I see a few discussions that have new posts. However the top one is greyed out, i.e. means I must have read it. But that's not the case, I haven't read the new posts in it. Indeed, I open it and it's not read, there are new posts there. Second time and the same thing happened and it's the same discussion. I remembered that this is a discussion that I renamed two times, as well as changed tags (as an admin) and then I deleted the automatic posts that contain text explaining I have renamed and deleted stuff. Is it possible that removing those auto-generated posts somehow messes up with the tracking of the number (?) of posts I have read in a discussion or something like that? This is a wild guess and I'm not sure if you mark the actual post ID I've read last or number of posts, but if for some reason it's the latter, is it possible that somehow you do a simple subtraction of post ID-s to count the number of posts I've read, however since I deleted some of them, the calculation is wrong? I know it's probably not like that and the ID-s are globally unique, not just for the discussion but I don't have any other explanation.
BTW, I've had problems with other discussions in the past where I deleted those auto-generated posts, like for instance some discussions with hundreds of answers in them. So, I move some posts and delete the auto-generated posts and then it seems that for a while whenever I open the discussion, I receive an error and can't open the discussion. But after a while it repairs itself when more posts appear there. It's a speculation on my side because I didn't troubleshoot it, nor recorded the actual message, however I have this feeling something is wrong with deletion of auto-generated posts (those that tell discussion was renamed, tags edited, etc.)