[copied from https://issues.civicrm.org/jira/browse/CRM-20711 noted on CiviCRM version 4.7.27]
In the Contact record, when I try to remove or delete a contact from a group they've already been added to, I receive this error:
DB Constraint Violation - group_id should possibly be marked as mandatory for GroupContact,get API. If so, please raise a bug report.
I am able to remove a contact from a group when in the Group's list of contacts view, so this isn't critical. Posting in case it happens for others.