Why is Lt warning that there are unanswered questions when I try to commit a lesson?

If a warning is displayed when attempting to commit a lesson in Lt, as pictured below,

Then check the following to ensure they have been completed:

• All questions on primary pages in the lesson
• All questions in popup windows or on background pages.
• Unit conversion tables (in select labs, usually located in popup windows)
• Graph question panels with unlabeled axes (see image below).