Is END_TYPE field problematic in calendar module as it is missing a condition for a calendar.event ?
Odoo says there is a mandatory field not filled in but this field should not be required as the recurring boolean has been deselected...
Odoo version 11, tested on runbot.
In Calendar Module, create a calendar.event
In the option tab, select Recurrent = true and chose end_type (UNTIL label) as END DATE
Now, unclick Recurring (=false), try and save the calendar.event and Odoo asks Following Fields Are Invalid : 'Repeat Until'.
It seems there is a constraint missing : if the recurrency boolean is false, then this field is not mandatory anymore.