This comes from 643538: we need to improve vg_validate to catch the situation arising in that bug, since the resulting metadata is corrupt. Basically, this amounts to checking that pvmove LVs are actually mirrors. In the bug, the pvmove LV becomes an "error" segment.
(It may also make sense to make sure they are 2 way mirrors.)
The code for this has been reviewed and checked upstream. Closing.