It began with an old drawing that looked like this. In the middle of that birds nest is an electrical layout with lighting blocks (AutoCAD predating our use of AMEP) and circuits.
The mess is the result of some blocks showing a massive, larger than building extents, outline. In the middle under that centre grip are the original lighting block graphics.
The outline is actually a fully functional XCLIP frame which somehow is associated with the block insertion. Explode the block & it disappears, redefine the block and it survives but is not replicated on any new insertions!
I tried redefining blocks by importing, even exploding and remaking the block in the file but a solution eluded me. Thanks to Rodney at Autodesk AU/NZ Support for discovering that it was actually rather simple to fix:
Select all the mutant “XCLIP” frames
This removes the frames leaving the original blocks intact.
I’m not sure if 2012 changed how the block was interpreted or, more likely, its default settings exposed an entity which had been lurking all the time. Whatever, it’s nice to have a clean file again.