Copy and paste from one page to another creates hidden macros

I copied a warn macro from the view page into the edit section of another page. It appeared that the macro had been lost in the paste, and so I copied it into a new macro.

The end result is that the block is inside two warn, and yet that doesn't show up in the editor - clearly there is a hidden block somewhere.

Of course if I could edit the source fixing this would be trivial, but this is yet another failing of the so-called wysiwyg editor.