I hit [Shift], clicked on two blocks in order to delete them, and as I moved them, I got that explosion of blocks from several surrounded blocks.
Normally I would just hit Ctrl Z to undo the damage, but this time it got 'sticky, and i could drop any blocks, so i was dragging around the clump you see in the pic.
My only option was to Log out, and log back into AI2, and this is what I have now. I don;t need a comment about Checkpoints, my last one was a fair stretch behind this ... incident.
Just beware, multiple bugs can go off at once, evidently