What the message means
The message means that the selection of values for attributes in the cascading drop-down is not valid according to the list to which it is connected. Normally the software does not allow selecting such values in the first place, so the message is not frequent in normal circumstances. The message appears anyway in the following circumstances:
The list was changed and the value chosen in a previous session is no longer valid. In this case, the information in the message box is to the point. From WebCenter 14.1 onwards, the message will be more elaborate (pointing out which field is not valid).
There is a difference between the Excel based cascading drop-downs and the database based cascading drop-downs when it comes to interpreting multiple spaces.
Excel-based: Multiple subsequent spaces are replaced by one space (so "American Samoa" is replaced with "American Samoa"). This is done by Microsoft Excel driver and Esko cannot change this behavior.
Database-based: "American Samoa" is kept as is (we just stick to whatever we get, which is what you would expect).
When now an attribute was saved from an Excel-based list and then a database-based list is used to update it (after migrating the list to the database or by mixing the two systems), WebCenter will see a difference between "American Samoa" and "American Samoa".
Making it worse, you can actually not see that there is a difference since HTML displays "American Samoa" as "American Samoa".
Unfortunately there is no any good solution: It is correct to warn that there is a difference since you are about to save different values to your database. This way mixing up matters further.
Remove all multiple spaces from the Excel sheets used (both from the front-end and from the Excel sheets you feed to the database). In case you want them anyway, use only database lists.
The message box is annoying but not blocking. Most browsers also have an option to suppress further appearance of the same message on the same page.