To replicate use example "web1".
Go into "MailboxedFormControl" and make "mai:MailboxName" a required field.
Now run the program, insert a new record, but leave the "Name:" field blank. Check "Forward Only:" and click "Save".
The error pops up. Now Uncheck "Forward Only:", enter something into "Name": and click save. The record will save correctly.
Go back into the record and you will see that "Forward Only:" is still ticked.
It only seems to happen on an insert. Note, the other fields on the form CAN be changed following the "validation" check. Only the checkboxes seem to be written in stone once Save is pressed.