There are some fuzzy corner cases in the way the Common Lisp standard describes the syntax and semantics of FORMAT directives. I have prepared a first draft of a clarification proposal for the relevant parts of the standard, which is attached in PDF format. I'm giving myself some time for comments and feedback before actually submitting this to the Common Document Repository.
Update: this document (in version 1.1) has been submitted and accepted today as a new item in the Common Document Repository. It can now be officially referred to as "CDR 7".