After upgrading to RC5 we began getting this error:
ngModel cannot be used to register form controls with a parent formGroup
directive.
Try using formGroup\
If component has more than 1 form, register all controls and forms
I needed to know why this was happening in a certain component and not in any other component.
The issue was that I had 2 forms in one component and the second form didn't yet have [formGroup] and wasn't registered yet since I was still building the forms.
I went ahead and completed writting both forms complete without leaving a input not registered which solve the issue.