Here's a great one.
Upgraded client from Exchange 2003 to Exchange 2010.
Purchased and installed UCC SSL Certificate from GoDaddy. Included external and internal server names on SSL Certificate.
SSL via Web works perfectly. Internally, users are getting errors in Outlook that the certificate doesn't match the server name. Turns out that's true. The internal Active Directory domain is abc.com which is a variation on the company name and a variation on their actual internet domain abc-inc.com. Unfortunately, they don't own abc.com and therefore have no authority to approve an SSL request that includes that name.