Just found this error being triggered when trying to load the Visual tab of the WordPress editor on the Notifications tab of a GravityForms settings page. Nothing about it is GravityForms specific so I imagine that it could be happening in other non-standard placements of the WP editor.

Uncaught ReferenceError: SalsaPressVars is not defined
editor_plugin.js?wp-mce-4401-20160726:60