Hamlet

As all know, except, needless to say, those who don't, in the opening scene of Hamlet, Marcellus, Barnardo and Horatio gather at night in fearful expectation of the arrival of the dead king's ghost, which duly arrives. Upon the ghost's appearance, Marcellus says to Horatio: "Thou art a scholar; speak to it, Horatio." Which Horatio duly does, though he admits that the ghost "harrows me with fear and wonder."
Most will agree Horatio would have been fully entitled to ask what particular difference his being a scholar made. It is doubtful the dead king's strange and recurring appearance was provoked by a special desire for learned discourse.
As for the ghost, when a scene or few later he speaks to his son, Hamlet, and incites bloody revenge on his murderous brother, his fears should have been raised as to Hamlet's prospects in the task when his startlingly immediate plan for killing the new king, utterly vague but for this point, is to decide to pretend to all to have gone mad. Hamlet's father should have asked what bloody good this would do.
Naturally, Hamlet's subsequent mad behaviour achieves nothing but for driving the wholly innocent and cruelly treated Ophelia genuinely mad with confused grief, and onwards to a suicidal, pitiful death. What else Hamlet supposed this display of madness might achieve is anyone's guess.

If one were so crude as to reduce Shakespeare's most famed work to a moral, it would be that if when entrusting some important task to someone his immediate instinctive suggestion towards completion of the task is to pretend to be insane, chances are you are backing a loser. More than likely he's simply looking for an opportunity, however inappropriate, to pretend to be mad, and doesn't merit much trust.