I was editing a post in Visual (not Text) and clicked abc. It threw a 501 error code. I just checked it to be sure I went through all the steps and it happened. The error is /wp-includes/js/tinymce/plugins/spellchecker/rpc.php not supported.
I never added a plugin to spell check so believe it is part of my theme. How do I, well, support it? :) Is it my host, my computer settings?