This normally happens when the first packet from a peer is received. Either peer delay or peer dispersion is greater than one second. Ya gotta be kidding. The following TEST5 through TEST10 ennumerate errors in the packet header. The packet is discarded without inspecting its contents.
Cryptographic authentication fails.
See the Authentication Options page. Peer is unsynchronized. Wind up its clock first. Peer stratum is greater than 15. The peer is probably unsynchronized.