Redmine

Hopefully the title makes sense.

1. Create Issue 1
2. Create Issue 2 and assign 1 as parent
3. Create Issue 3 and assign 1 as parent
4. Navigate to issue 3 and assign 2 as parent
5. Navigate to issue 3 and re-assign 1 as parent

Step 5 will display "Parent Task is invalid".

Workaround:
5. Navigate to issue 3 and remove any parenting
6. Navigate to issue 3 and re-assign 1 as the parent

Seems the issue can't be assigned to its parent because it would create a conflict reassigning the lft and rgt fields of the 3 issues involved.

Environment:
  Redmine version                3.0.1.stable
  Ruby version                   2.0.0-p598 (2014-11-13) [x86_64-linux]
  Rails version                  4.2.0
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.8.10
  Git                            1.9.5
  Filesystem