My ISP auto-updated all my WP sites to 5 a while back - all without a hitch - except that on one of them I noticed that an CSS control was not rendering on my posts as CSS but as straight text.

A little analysis revealed that Gutenberg had unilaterally decided to add some code prior to that tag, which was causing the problem.

Didnโ€™t happen on the other blogs, so what went wrong?

Calls to the ISP - tickets into WPMUDEV, searching bulletin boards … no luck.

I went in to admin and could edit the post and that was good - but this was going to take a looong time.

Enter Mars Edit.

Synch all posts and search for the offending term - get a list of broken posts …. edit post, save … repeat.

In the end it took me about 30 minutes to modify some 200 posts. A pain in the arse, but waaaaaaay quicker than using the WP admin panel. And I had definitely spent more time than that just researching the problem.

Massive shout out - yet again to @danielpunkass - love that product of yours.

As to what caused the issue? Still no idea. Don’t really care. Life is too short.