Recently, I had to trouble shoot an issue where the Ribbon was not being displayed for a particular entity. When navigating away from the form, the following error was given by CRM 2011:
<Message>Unable to get value of the property 'attrs': object is null or undefined</Message>
It turns out that this was because of a FlyoutAnchor/SplitButton with no PopulateQueryCommand or Menu Sections added.
The most recent version of the Ribbon Workbench now has a validation rule that checks you have either of these before publishing.