Redmine

1/ go to a project settings
2/ add a new member (note that there's an error if the role isn't selected)
3/ so choose a role
4/ edit the member again and this time remove all his roles
5/ this works, but there should be an error instead

this way you can create an invalid member that could lead to internal errors elsewhere

xhr doesn't show validation errors at all
https://github.com/redmine/redmine/blob/master/app/controllers/members_controller.rb#L93
member_roles are deleted even before actually saving the member
https://github.com/redmine/redmine/blob/master/app/models/member.rb#L77

I think it's a bug

Environment:
  Redmine version                5.0.3.stable
  Ruby version                   3.0.2-p107 (2021-07-07) [x86_64-linux]
  Rails version                  6.1.7
  Environment                    production
  Database adapter               Mysql2
  Mailer queue                   ActiveJob::QueueAdapters::AsyncAdapter
  Mailer delivery                smtp
SCM:
  Subversion                     1.10.4
  Mercurial                      4.8.2
  Git                            2.20.1
  Filesystem                     
Redmine plugins:
  no plugin installed