Notice: register_sidebar was called incorrectly. No id was set in the arguments array for the "Left Sidebar" sidebar. Defaulting to "sidebar-1". Manually set the id to "sidebar-1" to silence this notice and keep existing sidebar content. Please see Debugging in WordPress for more information. (This message was added in version 4.2.0.)....
For the longest time sidebars didn't need to use an 'id' when they were being created. As of WordPress 4.2, they have made it mandatory - everything will still work but they will issue these messages. This will only display if you have 'WP_DEBUG' set to 'true'.
You can correct it one of two ways (1) edit your wp-config.php and changing the line
This will be included in a future release.