  We construct two non isomorphic contractible affine threefolds X and Y with
isomorphic cylinders, showing that the generalized Cancellation Problem has a
negative answer in general for contractible affine threefolds. We also
establish that X and Y are actually biholomorphic as complex analytic
varieties, providing the first example of a pair of biholomorphic but not
isomorphic exotic affine 3-spaces.
