DEFVAR-INIT-TIMEDEFVAR is not completely clear about the time at which the initialization occurs.
On p68 it says ``VARIABLE is initialized to the result of evaluating the form INITIAL-VALUE unless it already has a value. The INITIAL-VALUE form is not evaluated unless it is used; this fact is useful if evaluation of the INITIAL-VALUE form does something expensive like create a large data structure.''
At least one implementation interpreted the "unless it is used" to mean "unless the variable is used" rather than "unless the initial-value is to be used". The problem is that the "it" is ambiguous. Thus, DEFVAR was interpreted as a kind of lazy initialization that happened upon the variable's first unbound reference. (This interpretation appears to have been further supported by the additional wording in CLtL about not creating expensive structures that are not needed.)
DEFVAR execution time (if at all). The cause of the confusion is the statement that the initial value form is not evaluated unless "it is used". Better to say that INITIAL-VALUE is evaluated if and only if the variable does not already have a value. Then there would be no confusion about the time of evaluation.