Had a weird one when setting up a site for a client.
On one particular post, whenever I saved to draft I would get:
Forbidden You don't have permission to access /appsol/wp-admin/post.php on this server. Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
I narrowed it down to being some text in the content.
They had some industry terminology like 'Energy Reduction System' and then would follow up with the abbreviation in Brackets.
I discovered that whenever 'System (ERS)' appeared in the text, it would throw the error. If I put a comma before the bracket, changed the bracket to quotes or anything, the post would save correctly.
It doesn't matter what's in the brackets... just the "System ()" which is obviously code that I'm not allowed to execute.
My client will be using this terminology regularly, so it would be great to fix this up.