When you are saving a Visit that has at least one VisitAttributeType, the values of the attributes that the user entried should be repopulated into the fields when redisplaying the field with an error.
Create 2 VisitAttributeTypes of boolean
Set the types to 1 min, 1 max
Try to create a new visit but don't select the value for one of them. Neither yes/no/true/false is checked when the form is shown again.
(didn't test whether string values are repopulated or not)