As reported by J.M., when dropping a user from the Users page, if one selects the "Drop the databases that have the same names as the users." checkbox, the confirmation message appears immediately. It probably should only be displayed when clicking Go.
Fixed the bug. Pull request :
https://github.com/phpmyadmin/phpmyadmin/pull/1199
Fix: https://github.com/phpmyadmin/phpmyadmin/commit/979adb93828a3e1b5d2ed89e903d7f6b35639b62