Thank you for the quick reply. It is the latest version of the plugin – all plugins and WP database are the latest versions. I haven’t touched the code on any of them. I will test it with different plugins turned off and see what happens.
I recently added a few including W3-TC and RS Head Cleaner Plus. I’m pretty sure the error didn’t appear until tweaking settings in those other two plugins, but I’m not positive about that. I didn’t notice the error as I was logged in as admin, I had a customer notify me. It only shows in other browsers and mobile where I’m not logged in to WP.