M-x load-library <RET> etags <RET>
M-: (tags-verify-table "any-file-not-tags") <RET>
It returns nil and leaves behind a buffer visiting the file, in Tags Table mode. Not nice, but more or less as expected. Not a bug.
However, if you verify a (no tags) file already being visited in a buffer, it returns nil and *leaves* the buffer in Tags Table mode!
Well, you'll say, "don't do that". Ok, except that you can do it by accident, for example if you're using bs-show and happen to type "t" in the wrong line.
Which, arguably, could be fixed in bs (though I think it's a bug in etags.el, not bs.el), except that I don't think there's a tags-table-buffer-p or equivalent to check that the buffer contains a Tags Table. The way to test it seems to be `tags-verify-table'...