Using WP-debug to troubleshoot an issue with a plugin, I was getting an error that read something like:
Notice: Undefined index: page in DIR\functions.php on line ###
and
WARNING: wp-includes/functions.php:### – Cannot modify header information – headers already sent by (output started at DIR/vslider.php:###)
The “Notice” issue can be resolved by adding following these directions: https://www.paulcracknell.net/434/notice-undefined-index-page-in
The “Warning” issue can be resolved by applying the same fix as the “Notice” issue as well as checking for something known as the “Whitespace Problem” which is described here:
https://www.geeklog.net/faqman/index.php?op=view&t=38
If you happen to be using vSlider, and you are encountering the same debug errors, replace your “functions.php” file in the folder /wp-content/plugins/vslider/
with this file (zip compressed): vslider.php