Under "Application Access" in the admin panel, attempting to remove a group that currently has the "default" column checked results in a declined XHR (status 412 precondition failed).
For example, with two groups present for an application, both set to "default", the outgoing payload is quite straightforward:
{{
{"groups":["jira-administrators","jira-users"],"defaultGroups":["jira-users"],"selectedByDefault":true}
}}
The response received:
{{
{"errorMessages":[],"errors":{"versionMismatch":"The page is out of date, please refresh the page."}}
}}
The page is then refreshed, but nothing changes.
This was tested under Jira 7.8.1