EDIT: 9/19 – this is a known issue and will be resolved in the Office 2010 client December 2013 updates. 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.
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