Smart List Validation Error
Issue
You encounter Validation Errors in the Smart List.
Solution
If you hover your cursor over the red squiggle in the Smart List, Marketo should pop-up a reason why the error is occurring.
A very common reason for Validation Errors when using an "is" logic for a constraint is that the value being used does not exist in the database.
In situations where you are uncertain if the exact value exists in the database, try using "contains" for the constraint logic.