I also noticed that the Stretch To Full Width image on the comic page had also suffered an identity crisis. There are 2 images in that particular block, Whaam and Speedlines. Speedlines was set to the back and was stretch full width. In the main variant, all was as should be, in the mobile, the images had swapped places. Whaam was now at the back and stretch full width, with speedlines having no stretch and at the forefront.

Fixing it was easy, but it looked fine in the project. The problem was only evident when previewed or published. Like a messy stain under black light, undetected unless you go to the trouble of scrutinising.

I've added a screenshot as I've now fixed the (this) error.

Click image for larger version.  Name: Blocks Mess.jpg  Views: 71  Size: 91.0 KB  ID: 134749

I had written the above, well, something similar, and clicked the 'Insert Image' icon when the page went unresponsive. 'Change your password' was screamed across the screen. 'We don't care that you think you've been insightful. We don't care how long you took to write your words. Let them wither and die like grapes on the vine, putting a '1' at the end of your existing password is THE single most important thing in the Universe right now. Do it, peasant, or be banished. Now get out of my sight.'

So I changed my password and re-wrote it.