EDIT: 9/19 – this is a known issue and will be resolved in the Office 2010 client December 2013 updates.
EDIT2: 12/13 – the fix did not make it into the December 2013 update and the product group currently has no ETA on when this fix will arrive.
EDIT3: 1/17 – Hotfixes are available below, or you can wait until the February 2014 Cumulative Update.
If you have a custom content type with a multi-line site column added to it, and the client has Word 2010 SP1 or SP2 with KB2760758 installed, attempting to save the document will yield an error:
If you only enter a single line of text in the multi-line document property, the save will succeed. Uninstallation of KB2760758 should also work. To validate which version of MSO.DLL is installed, look at C:\Program Files (x86)\Common Files\Microsoft Shared\OFFICE14\MSO.DLL. If the file version is 14.0.7106.5001, the issue is present. Earlier versions, such as the SP2 MSO.DLL, 14.0.7015.1000, do not have this issue.
KB Article Number (s) : 2878218