Surprisingly, this displays "not found":
(let ([h (make-hash-table)])
(hash-table-put! h "some-key" #t)
(if (hash-table-get h "some-key" (lambda () #f) )
(display "found")
(display "not found")))
That's because MzScheme hash tables use `eq?' for hashing, by default.
In the 199.x versions, you can use the 'equal flag to change the
behavior to suit your taste.
-- Paul