After deleting a child folder from an IMAP account, the parent folder's subtree is collapsed on the navigation pane, and any sibling folders will not be visible on the navigation pane until you have collapsed and then expanded the grandparent folder/account on the navigation pane treeview.
Moderately severe - this bug caused me to delete an important email folder from my main work email account, thinking it was empty because no contents were shown. If this is causing problems for computer experts, I can only imagine what will happen when novices start using Outlook 2010.
100% repeatable. Just follow the procedure described above.
Potentially similar issue: