The module Modal Frame CCK Editor transforms field type to value when field(s) are not being edited via the modal frame. Required Date fields then fail date_combo_validate in date_elements.inc with the error:

The dates are invalid.

To reproduce,

1. create a node with a required date and a text field
2. enable modal editing on the text field
3. edit the text field, you will see the validation error on the date field

Cross-referenced issue and patch