After removing a field from a form using $removeControl, if you change the field's value [to be invalid] it will affect the validity of the form it was removed from.
Not too sure if this is working as expected or not, but seems to be undesired.
Affected versions [at least] 1.3.x, 1.4.x

Steps to reproduce (given the following plnkr)
http://plnkr.co/edit/NZJn0zwWdKxvrIlQxUmn?p=preview

1. Add an invalid value into the field (it accepts numbers).
2. Note the validity of the form and field. (Form: invalid; Field: invalid)
3. Click the button to remove the control via $removeControl(...)
4. Note the validity of the form and field. (Form: valid; Field: [not on formCtrl])
5. Edit the field to be invalid.
6. Note the validity of the form and field. (Form: invalid; Field: [not on formCtrl])

For step 6 I would expect the form to no longer be affected by the field, especially since step 4 set the form to valid when the invalid field was removed.