1  Exercise 3.41.  Ben Bitdiddle worries that it would be better to implement the bank account as follows (where the commented line has been changed):
2 
3 (define (make-account balance)
4   (define (withdraw amount)
5     (if (>= balance amount)
6         (begin (set! balance (- balance amount))
7                balance)
8         "Insufficient funds"))
9   (define (deposit amount)
10     (set! balance (+ balance amount))
11     balance)
12   ;; continued on next page
13 
14   (let ((protected (make-serializer)))
15     (define (dispatch m)
16       (cond ((eq? m 'withdraw) (protected withdraw))
17             ((eq? m 'deposit) (protected deposit))
18             ((eq? m 'balance)
19              ((protected (lambda () balance)))) ; serialized
20             (else (error "Unknown request -- MAKE-ACCOUNT"
21                          m))))
22     dispatch))
23 
24 ;; because allowing unserialized access to the bank balance can result in anomalous behavior. Do you agree? Is there any scenario that demonstrates Ben's concern?