Everyone has made silly mistakes as a forum owner, which you learn by and move on. Mine personally was not taking backups in the early stages of my previous forum, and we were on about 30k posts I believe and we were hacked and that was it. Had to start from the very beginning again, and as most people know the beginning of a forums life is pretty important.
I've made a couple more, ones like forgetting to remove unused plugins and installing too many, which has resulted in my current forum being compromised once.
What's yours?
I lost a few months of posts due to no backup. My vanished without notice or trace and, with no backups, the most recent version of my forum was the one on my previous host from several months before.
Same here with the backups. A current forum that I've been a part of since 2006 lost its database in 2011, which resulted in losing about 60k posts. That same forum was also moved several times and started off on InvisionFree. Which is already a rookie mistake to begin with.
EDIT: Then again, I wasn't the founder, so I didn't start it on IF.
Backups here, too. I didn't have control of Otaku Space's backups until it went down last July. I lost 2500-some posts, and all the momentum that was getting going. So, I had to start it from scratch. I didn't want to give up on the project, though, so it's still going (slowly).
Not changing my password. Lol.
My biggest mistake was opening the forum in an unpolished state. I had content (I knew you needed that) but I used a theme I had made myself (oh dear) and had forgotten to enable registrations.
As a result I got a rep in the Sims community as that odd forum owner who does not want members and seems to like to torture users with black text on black backgrounds (Opera auto adjusted the contrasts for me). What a noob.
Things I learned: Check your site in as many browsers as you can. Make sure the place is looking SHARP before you launch.
Mine was once accidentally typing something like rm -rf /* instead of rm -rf ./* while logged in as root... I had recent backups but they were all local... By the time I figured it was taking a bit too long to delete a few dozen log files it was too late.
I learned not to make a habit of using the root user, keep external backups and double check any destructive operations... I also now always use full paths whenever using rm rather than relative paths.
I'll jump in the bandwagon and say lack of backups and cheaping out on hosting.
Uh oh. I guess I am making the backup mistake right now. *walks away to hide*
But I think my biggest regret in the last 2 years is not starting with MyBB. When I used phpBB, I had momentum, then I lost it when the merge system broke down on me. I lost probably some 400 good posts.
Mine would definitely be my choice of hosting. I chose the cheapest host I could find and paid for a year straight up. I ended up leaving them after a month.