Major I had a cluster running 1.5, which I then downgraded to 1.4 and ran for a while (including tablet copies happening). I then upgraded back to 1.5, but the tservers would not start.
The issue is that the tablet-meta files on 1.5 had gotten invalid path IDs written into them during tablet copies that occurred on 1.4. The 1.4 servers had copied a remote TabletMeta file, and preserved the unknown protobuf field and serialized it locally. Then when we upgraded again, they failed to validate since they didn't match the local path UUIDs.