Recently my laptop crashed while Microsoft Outlook was open which caused one of the open .PST files to become corrupted. The PST file that was corrupted was for a Gmail account that was synced with Google Gmail via IMAP. The error message that popped up when attempting to expand the folders is described below and then displayed in the image below the explanation.
Error: The set folders cannot be opened. Errors have been detected in the file C:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook\OutlookAccount@gmail.com-00001.pst. Quit Outlook and all mail enabled applications, and then use the Inbox repair tool (Scanpst.exe) to diagnose and repair erros in the file. For more information about the Inbox repair tool, see Help.