This post is more of a note to myself but may save someone else a lot of wasted time trying to track down why they can no longer save selected nodes in Umbraco. In a recent project I created a number of custom user controls for the Umbraco Dashboard, these user controls contained validations controls but I had overlooked adding the validationgroup property on the validation controls so when attempts were made to save or publish a node containing one of these user controls that didn't validate, the changes would not save as the page could not be posted back to the server. The problem is not always immediately obvious if the user control that is not valid is on a different tab to the one you are on when attempting to save or publish.
I guess an alternative or complimentary method would be to display a Javascript alert notifying the user that there are still controls that don't validate.