If you bulk update a lot of issues like when moving some to a newly created project an email is sent for each one. I know that one can turn this off globally in the administration section (and then back on after), however I think that this should be either a permission (disable notifications on bulk update) with a check box on the bulk update screen to disable them or putting all of the update information into one email. I believe that either or both options would be good additions to redmine.