Huh?
Hell, between cache clearings the Views menu link itself (under admin/structure) disappeared.
But actually the error was all me.
Wow.... it was the contextual filter i had added (thinking i was just adding it to the block and accidentally adding it to all displays).
If you give a page a default filter of use the content ID from the URL, and there is no content ID in the URL, rather than being the view page with no items you get page not found. Not the most helpful result from an easy-to-make (for me) error.
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