I realised after posting the description may not have been the clearest... However when I went back in to reproduce this and take screenshots, I couldn't, as the offending item had disappeared. Along with several others.

 

The simplest I could put it is: I made a new grid, which should of course have been blank. Instead, it contained an item, with Item Text "Header 2". This item also had text in the HTML Pane. This latter was identical to text in the HTML Pane in another item in another grid.

 

Deleted that new grid, created another. Same thing. Should have been blank, instead had this weird "Header 2" in one of its items. And the same text as before showing in its HTML Pane. I'm saying "same text", I think really it was just the same HTML Pane.

 

Later, in that other grid, I added about 70 subitems to an item. Now when I created a new grid I not only got all of the above extraneous stuff appearing, I also got  c.70 subitems.added to the "Header 2" item.- although these subitems in this new grid were blank.

 

Wanting nonetheless to use this new grid I had created, I deleted all this stuff that had appeared - the "Header 2" item and its 70 subitems, the text in the HTML Pane.

 

Now when I went back to the other grid - that item, with all its subitems, had gone.

 

That's it. Obviously there was some sort of connection between this item in this existing grid, and any new grid I created.

 

Luckily I'm still at a "trying out" stage, so there's no great loss in this. But it's annoying and it does kind of interrupt the workflow. (Now that I've deleted all that stuff from the new grid none of this happens anymore. New grids are properly blank).