Section 13 implies that a precondition failure is a good safe mode. However, a precondition that isn't recognized by a server will not share this property.

Regardless, if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates to false.

A server that does not recognize a header as a precondition will ignore it and likely proceed with handling the request even if the precondition expressed should have failed.

Indeed, the same is true if a server ignores preconditions, though the specification effectively mandates that they be supported.

Section 13.2.2 is the only text I could find on defining new preconditions.

(One thing I really want to avoid is server's applying special heuristics to headers that start with If-, because that is one of many poor outcomes if this is left.)