12/9/14 Update: 2013 CU7, 2010 SP3 UR8, & 2007 SP3 UR14 have been release:
https://blog.jasonsherry.net/2014/12/09/exchange-2013-cu7-2010-sp3-ur8-2007-sp3-ur15-released/

9/16/14 Update: Chrome 37 issue added
9/2/14 Update: More known issues added
8/31/14 Update: Do NOT install 2013 CU6, when co-existing with Exchange 2007, until you have this hotifx: KB2997209 or if you have or want to setup a hybrid relationship with O365 until you have KB2997355. See known issues for more details.
8/29/14 Update:  Known Issues added below. Microsoft released new updates packages for Exchange 2013 CU6 (2961810), 2010 SP3 RU7 (2961522), & 2007 SP3 RU14 (2936861) today. For full details see the Exchange Team’s EHLO post: http://blogs.technet.com/b/exchange/archive/2014/08/26/released-cumulative-update-6-for-exchange-server-2013.aspx

2013 CU6                   Download | KB2961810 | EHLO Post
2010 SP3 RU7            Download | KB2961522 | EHLO Post
2007 SP3 RU14          Download | KB2936861 | EHLO Post

Like all CUs for 2013 this one is also a full version of Exchange that can be installed as a 1^st time install or will upgrading an existing install of 2013. Key updates in 2013 CU6 are the increase of Public Folder scalability to 100,000 folders, more info here, and a fix for the issue with the Hybrid Configure Wizard (HCW) failing on 1^st run or when attempting to modify the settings, see KB2988229 for more info on the HCW issue. Like many of the CUs, CU6 also updates the AD schema. For more info on 2013 CU6 see Tony Redmond’s post here: http://windowsitpro.com/blog/exchange-2013-cumulative-update-6 Like all CU/RUs these contain the recent fixed and security updates, in additional all three contains the latest DST updates. I will add a known issues section to this post when I come run into or hear about issues with these updates.

2013 CU6 Known Issues:

• 8/31: CRITICAL: Breaks administration inEAC: Unable to create newO365 based mailboxes, unable to move mailboxes toO365, and unable to create In-Place Archive mailboxes after installingCU6
  • For more details see Jeff Guillet’s post here: http://www.expta.com/2014/08/dont-deploy-exchange-2013-cu6-if-youre.html
  • Fix\Workaround: 9/1: Microsoft has released KB2997355 that includes a script that fixes EAC, via an update to the RemoteDomains.xaml configuration file. You can download the script directly here,
  • AlsoseeTony Redmond’s posts
    • About the fix: http://windowsitpro.com/blog/microsoft-fixes-hybrid-connectivity-problem-exchange-2013-cu6
    • About the orginal issue: http://windowsitpro.com/blog/exchange-2013-cu6-hybrid-and-co-existence-bugs
  • There is also issue with CU5 and Hybrid, due to a change in Exchange On-line customers running 2013 CU5 or earlier will be unable to finish the Hybrid Configuration Wizard (HCW), it will thrown an error.
  • There is an IU for this issue too, KB2988229 and http://blogs.technet.com/b/exchange/archive/2014/07/30/important-update-available-for-exchange-server-2013-hybrid-deployments.aspx 
• 8/29: CRITICAL: When co-existing with Exchange 2007, if a 2007 based user triesto access a 2013 mailbox, and maybe under other scenarios, the Information store process (Microsoft.Exchange.Worker.Store) for the target mailbox database on 2013 will crash.
  • Fix\Workaround: You must get the IU KB2997209 installed on 2013, after installCU6, to fix this issue
    • You have to call PSS to get this IU
• 9/2: HIGH:OWA bug doesn’tpreserve items under litigation and in-place holds
  • Some details in KB2996477
  • Tony Redmond posted about this issue here:
    • http://windowsitpro.com/blog/owa-bug-compromises-item-preservation-litigation-and-place-holds
    • http://thoughtsofanidlemind.com/2014/09/05/reporting-delegate-access-to-exchange-mailboxes/
  • The issue, where a delegated user can delete items that are under litigation hold, seems to have existed since 2013 RTM and still exist in CU6 and Exchange Online.
  • Workarounds:
    1. Remove delegated access to mailbox under litigation hold
    2. Disable OWA access to mailbox under litigation hold
    3. Put all users who have been delegated access on litigation hold also
• 9/2: HIGH: Exchange ActiveSync (EAS) access to mailboxes on 2007 fails
  • So Exchange 2007 based mailboxes will be unable to access their mailbox if 2013 is acting as the CAS\proxy for it.
  • More details on Tony Redmond’s post here: http://windowsitpro.com/blog/exchange-2013-cu6-bug-causes-activesync-failures-exchange-2007
    • Thread on TechNet forums here
  • Workarounds:
    • Get IU from Microsoft, see KB2997847
      • This update fixes this issue both 2007 issue, list here.
    • Configure Exchange 2007 users to use the FQDN that points directly to the Exchange 2007 servers, if they are Internet facing
• 9/16: HIGH: Chrome version 37+ break some functionality inOWA & ECP
  • After Chrome (32 or 64-bit)is updated to Version 37 some dialog boxes will not longer open
    •  This is due to Google dropping the showModalDialog method in Chrome. For more details see Tony Redmond’s post here: http://windowsitpro.com/blog/google-kills-showmodaldialog-api-chrome-37-and-does-evil-exchange-owa
    • It’s highly doubtful that Google will fix this issue, so Microsoft is going to have to come out with updates to Exchange 2010, 2013, and EXO to update the OWA & ECP code to no longer use this API.
  • Workaround: Have users use IE or Firefox
• 11/6: High: Unable to move mailboxes from Exchange 2007 to 2013CU6
  • Move fails with [Move synchronization fails with the Recipient “<GUID>’ isn’t a mailbox”] error
    • Heard about this on the Exchange 2013 Info Sharing Group, I have not reproduced.
      • Bug has been confirmed by Microsoft per post in that group and post on dell.com support KB below
    • Also documented here: https://support.software.dell.com/pt-br/migration-manager-for-exchange/kb/133684
  • Workaround: Unknown, but should be fixed in CU7
• Known calendaring issues with iOS 8.*: http://support.microsoft.com/kb/3015401
• Current issues with Microsoft Exchange ActiveSync and third-party devices: http://support.microsoft.com/kb/2563324