When creating an Event record in CRM for iPad, the following error appears instead of the Validation Rule error message: An error has occurred.
While the same works in CRM Online, the Validation Rule error messages display correctly on the Event Edit page.
The Error Location field, on the Validation Rule, is set to Top of Page.
Next to Error Location, enable the Field radio button in order to display Validation Rule messages on the iPad.
CRM Help Documentation: Offline Validation Rules