d = {ord('f'): 'eff '}
for i in range(100):
....s = u'f' * i
....print i
....s.translate(d)

On my Panther laptop, that fails at i=25 with a MemoryError:

*** malloc: vm_allocate(size=1677725696) failed (error code=3)
*** malloc[2346]: error: Can't allocate region
Traceback (most recent call last):
..File "<stdin>", line 4, in ?
MemoryError

It works fine on the Linux and BSD boxes I was able to test on, 
though.