This is similar to earlier exercises.
Assume two processes P1 and P2 are trying to acquire same mutex object
Initially lets assume the the value in cell of the mutex is false.
Now in parallel execution both procedures can reach this line
(begin (set-car! cell true) in proc
test-and-set because the value of cell is false.
Now, again both of them sets it to true happily and believe it an exclusive access - Truth be told - they cant be more wrong :)