It's a task ownership issue. I've noticed that if I create a task, then put it in Common Tasks, and another user ever starts that task, THEY become the owner (KINDA).
If you get properties on that common task, the owner will show as YOURSELF grayed out, but when you UNCHECK the "Show in Common Tasks" box, you'll notice the owner changes to the person who last ran the task.
At this point, set the owner back to yourself, and RE-CHECK the "Show In Common Tasks" box and click SAVE. The task will reappear somewhere in the folder structure you are trying to delete!
So, this is a lot of work depending on how many tasks are causing the problem, but click "ALL TASKS" and sort the list by owner. Set yourself as the owner on all of them using the procedure above, all your tasks will reappear in the undeletable folder. Move them all somewhere else (I created a _COMMONTASKS folder in my tasks to keep everything organized) and now you can delete your undeletable folder!
Another old ticket I forgot to update and close...
I called support on this one and found that the database had some corruption. It wasn't that the task was assigned to a different user. I don't quite remember if we were able to run some SQL commands to fix this, but I ended up getting a new server and migrating our environment to a new clean SQL 2008 database.