A user called in, complaining that they were unable to create a new folder in a shared mailbox. The error message they got indicated that the folder already existed. I started troubleshooting the issue, and it transpired that they had created the folder already, though it did not show up. I tried a number of fixes, including restarting Outlook, removing the mailbox from account settings, and even deleting their mail profile altogether. It was this last attempt that would lead me to the correct solution.