5.1.1.1.1 Examples of Evaluation of Subforms to Places

 (let ((ref2 (list '())))
   (push (progn (princ "1") 'ref-1)
         (car (progn (princ "2") ref2)))) 
>>  12
=>  (REF1)

 (let (x)
    (push (setq x (list 'a))
          (car (setq x (list 'b))))
     x)
=>  (((A) . B))

push first evaluates (setq x (list 'a)) => (a), then evaluates (setq x (list 'b)) => (b), then modifies the car of this latest value to be ((a) . b).