RFC 7931, "NFSv4.0 Migration: Specification Update", July 2016

Errata ID: 4822
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: David Noveck
Date Reported: 2016-10-06

Section 5.8 says:

Note that the NFSv4.0 specification requires the server to make 
sure that such verifiers are very unlikely to be regenerated.
Given that it is already highly unlikely that the clientid4 XC is 
duplicated by distinct servers, the probability that SCn is 
duplicated as well has to be considered vanishingly small.  Note 
also that the callback update procedure can be repeated multiple 
times to reduce the probability of further spurious matches.

It should say:

Although the NFSv4.0 specification requires the server to make 
sure that such verifiers are very unlikely to be regenerated, 
different servers may use the same approach to the construction 
of such verifiers, raising the probability that two distinct 
servers might inadvertently assign the same verifier value. The 
fact that the servers in question have assigned the same 
clientid4 may raise this probability.  In order to guard 
against the possibility that such assignments might cause two 
distinct servers to be incorrectly considered the same, 
the SETCLIENTID procedure mentioned above needs to be repeated.  
Repeating the procedure once  is sufficient to ensure that the 
successive confirm values SCn, SCn'  generated by these repeated 
SETCLIENTID operations cannot all collide with a verifier 
previously received by the client when communicating with IPn.

Notes:

It appears that the original text underestimated the probability inadvertant of duplication of clientid4's and verifiers. Existing servers while conforming to to RFC7530, may generate the same sequence of clientid4's when they all happen to be rebooted at the same time. The new text deals with this possibility and ensures that two different servers cannot be considered the same.

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