mysql collation: utf8_general_ci
['a','u','č'] should be sorted as ['a','č','u'] but because of ruby sort it's reordered back as ['a','u','č']

  def assignable_users
    users = project.assignable_users(tracker).to_a
    users << author if author && author.active?
    if assigned_to_id_was.present? && assignee = Principal.find_by_id(assigned_to_id_was)
      users << assignee
    end
    
    users.uniq.sort
  end

I can provide a patch, are you interested or is it desired behaviour?

Environment (not important, all redmine versions and databases are affected):
  Redmine version                3.3.3.devel.16557
  Ruby version                   2.1.5-p273 (2014-11-13) [x64-mingw32]
  Rails version                  4.2.8
  Environment                    production
  Database adapter               Mysql2
SCM:
  Subversion                     1.9.5
  Git                            2.11.0
  Filesystem
Redmine plugins:
  no plugin installed