Section 3 states
Once the last element in a sequence has been returned, or the enumeration context has expired, the enumeration context is considered invalid and the result of subsequent operations referencing that context is undefined.

Section 3.2 (request) states
If the enumeration context is not valid, either because it has been replaced in the response to another Pull request, or because it has completed (EndOfSequence has been returned in a Pull response), or because it has been Released, or because it has expired, or because the data source has had to invalidate the context, then the data source SHOULD fail the request, and MAY generate the following fault

Section 3.2 (response) states
Additionally, once this element is returned in a Pull response message, subsequent Pull requests using that enumeration context SHOULD generate a wsen:InvalidEnumerationContext fault message; in any case, they MUST NOT return a valid PullResponse.

This appears to be inconsistent as section 3 states that it is undefined, the first part of section 3.2 states that the server SHOULD fail and MAY return a particular fault, later section 3.2 states that the server SHOULD return this fault and MUST NOT return a valid PullResponse.  

Proposal:
section 3.2 (response) be changed to: MAY generate a wsen:InvalidEnumerationContext message, and delete the in any case, they MUST NOT return a valid PullResponse.