It first happened when looking at a client’s site – after updating a post (or creating a new one), the ugly, dreaded message, “Warning: Cannot Modify Header Information — Headers Already Sent” was all over the screen!
Sometimes, you may encounter an error that goes along the lines of: “Warning: Cannot modify header information — headers already sent by (output started at /path-to-something-directory/some-config-file:xyz )”.
Although it looks bad, it can really be an easy fix. Maybe.
The good folks over at WPMUDev have a great article to describe and show how to correct this in great detail (along with 9 other errors that pop up with WordPress from time to time).
Not to leave you in a lurch, this error happens because you have some blank spaces or lines in one of the WordPress files – All you have to do to correct it is editthe file that is mentioned in the error message, and delete the bad whitespace! (Always make a backup of any file you create BEFORE you start editing it!)
If you want more details, check out WPMUDev.