Big mistake I made about 2 years ago on the forum and lost about 200 topics, roughly 1000+ posts...
This was when it was just a single forum for coal and I had the others that never got used. Anyhow I decided to split the single coal forum up into different categories like I have now and get rid of the unused ones. So first rule is when doing major changes to forum you do full backup etc. Made my backup and proceeded to rename a few of the old forums. I moved the appropiate posts from the single forum into the newly renamed forums. Checked everything out and everything looked great, functioned no issues... So after about a day I made new backup and deleted old one.
Anyhow I think it might have been a day or two later I notice there is very few topics in the stoker forum... Can't find them, no where to be found.
Here the forum that I had renamed was one I was using for a classifieds section. One of the features of phpbb is you can set it to autoprune forums, for example you can set it to delete threads after a week if there is no replies which is what I had that particular forum set to so I could just leave it be and it would clean itself out.. So the second i moved those posts into that forum most got pruned, I didn't notice it at first. Wouldn't have been a big problem except as mentioned I overwrote the backup that did contain them. The backup I did have was made after they were deleted.
So the moral of the story is have a backup for the backup. Now I have daily backup each day, I keep a weeks worth of them. An then I 1 for each month. Haven't gotten to it but at some point i'll have a one hour backup.