- Exchange Server 2013 databases unexpectedly fail over in a co-existence environment with Exchange Server 2007 - http://support2.microsoft.com/kb/2997209
- You cannot route ActiveSync traffic to Exchange 2007 mailboxes after you upgrade to Exchange 2013 CU6 - http://support2.microsoft.com/kb/2997847
- Hybrid deployment's centralized mail transport doesn't work (when you want to route all Internet bound emails - including users located in Office 365 to your on premise Exchange Server 2013) - http://nathennod.blogspot.com.au/2014/10/hybrid-deployment-route-all-emails.html
First 2 problems are currently fixed via an combined interim update "Exchange2013-KB2997209_2997847-x64-en.msp", and the third problem currently only has a workaround. Please contact Microsoft directly for the patch - the support call is free of charge.
Regarding problem one, even though we have migrated everyone across to Exchange 2013, the ExchangeIS keeps crashing at soon as the users logon to their mailbox (not after the mailbox is migrated). This is particularly annoying as you wouldn't know that you have the problem until users actually start accessing their mailbox.