There is a cute joke version of this, that uses an ambiguity in English to mask a logical error:
We want to prove all ravens are black. To directly confirm this, we would have to examine all ravens, which is not possible. Instead we show the contrapositive: all non-black things are not ravens. To prove it, we need only point to a single non-black object that indeed is not a raven, for example a white sheep. QED
(of course, this would work better if the above w/u had not made the reader more sensitive to the difference between "not all X are Y" and "all X are not Y".)