When protecting the administrator folder in Joomla using Apache htaccess to increase the security of the website (which by the way is a good thing to do to prevent too interested individuals to peek where they shouldn’t), you might get an error 404 message when trying to access administrator and the expected password prompt never appears.
The cause can be that the server is configured to allow user defined error pages stored in for example the error folder. If some of the error files are missing the above mentioned problem can occur. Make sure you have the correct folder name and file names for your error files.
This particular problem occured for me when I moved a Joomla site from one hosting provider to another and by mistake also replaced the error folder with the one from the old hosting provider which had a completely different file name structure for the error message files.