Why I see the word "undefined" instead of the real contents of article?

In versions before 4.3.0 it is caused by PHP setting "magic_quotes_gpc" is set to "on", it should be "off" for proper operation.

Setting "magic_quotes_gpc" is irrelevant starting from version 4.3.0.

Technical FAQ

Why isn't there any buttons in the editor section? / The editor buttons do not work with Firefox 3 / Sending e-mails using feedback forms or mailinglists fails / Why do I see strange symbols when I open install.php? / Why do I see a blank page when I open install.php? / Why do I see a blank page after a successful installation? / Why I am sent back to Step 1 from Step3 in install.php? / Why install.php hangs during Step4? / How can I change the database connection data? / Setting the database port to anything else than 3306 doesn't work / Why do I get an error if I try to add pictures or files into article? / Why I can't upload big files in File Manager? / Why I can't download big files in File Manager? / Why I see the word "undefined" instead of the real contents of article? / When I open an article in the editor-window, Window Installer popup appears / I get "500 Internal Server Error" when opening install.php / I get error "Unable to connect Saurus License Server!" / I get "ERROR! This license key is already used by another website!" / I get "Error! Receiver e-mail in mail form has been changed by unauthorized persons." / I have defined the right character set, but still wrong symbols are displayed. / Why doesn't my alias work? / How to make a private key to use for the E-commerce module? / I get error "Page not found" after logging into the admin area. / Backslashes "\" appear in articles after saving / The download dialog opens several times after one another / I get message "Due to technical difficulties this site can not be displayed. Check back soon!" / What does "open_basedir restriction in effect" error in Error Log mean?