Headers Already Sent

Installed and activated on WP 4.0.1. Immediately upon activation, the dashboard lit up with errors: headers already sent, and over 4,000 lines of unexpected output. Not sure if it’s worked well in previous versions or not, but it needs some work for 4.0.1.

https://wordpress.org/plugins/google-maps-ready/