If you recently upgraded to WordPress 3.3.1 like me, you may notice that you may be getting this error message below:

Fatal error: Call to undefined function is_rtl() in /home/user/public_html/wp-includes/general-template.php on line 2102

Or something similar. For me it was a pretty bad error: while the website loaded fine, I was unable to access the WordPress admin interface.

Though it seems like a problem with the WordPress core, after a bit of research I found others with the same problem and it all pointed to one plugin: Popup Domination. So if you have that plugin installed, either rename the folder or just delete it altogether and wait for an update. After deleting the folder popup-domination in my wp-content/plugins folder the problem went away.

Recap

If you encounter this error after upgrading WordPress to 3.3.1:
Fatal error: Call to undefined function is_rtl() in /home/user/public_html/wp-includes/general-template.php on line 2102

Delete or rename your popup-domination plugin folder to deactivate it.