A recent historical survey reports that men who've undergone a penectomy will occasionally experience a phantom John Thomas, and it's usually not a painful experience. In fact, some men will experience phantom erections and illusory ejaculation. What a silver lining!
Here's an excerpt of the abstract of "Phantom Penis: Historical Dimensions" by Nicholas J. Wade and Stanley Finger from The Journal of the History of the Neurosciences:
The historical record reveals several cases of phantom penises dating from the late-eighteenth century and the early-nineteenth century. These cases, recorded by some of the leading medical and surgical figures of the era, are of considerable historical and theoretical significance. This is partly because these phantoms were associated with pleasurable sensations, in contrast to the loss of a limb, which for centuries had been associated with painful phantoms.
The report also delves into the notion of the masturbatory mirage. The 18th cenutry surgeon John Hunter noted:
A serjeant of marines who had lost the glans, and the greater body of the penis, upon being asked, if he ever felt those sensations which are peculiar to the glans, declared, that upon rubbing the end of the stump, it gave him exactly the sensation which friction upon the glans produced, and was followed by an emission of the semen.
You can read more about this scintillating topic over at Mind Hacks.