An engineer who was unemployed for a long time decided to open a medical clinic. He puts a sign outside the clinic: "A cure for your ailment guaranteed at $500; we'll pay you $1,000 if we fail."

A Doctor thinks this is a good opportunity to earn $1,000 and goes to his clinic.

Doctor: "I have lost my sense of taste."
Engineer: "Nurse, please bring the medicine from box 22 and put 3 drops in the patient's mouth."
Doctor: "This is Gasoline!"
Engineer: "Congratulations! You've got your taste back. That will be $500."

The Doctor gets annoyed and goes back after a couple of days later to recover his money.

Doctor: "I have lost my memory, I cannot remember anything."
Engineer: "Nurse, please bring the medicine from box 22 and put 3 drops in the patient's mouth." Doctor: "But that is Gasoline!"
Engineer: "Congratulations! You've got your memory back. That will be $500."

The Doctor leaves angrily and comes back after several days, more determined than ever to make his money back.

Doctor: "My eyesight has become weak."
Engineer: "Well, I don't have any medicine for this. Take this $1,000," passing the doctor a $500 note.
Doctor: "But this is $500..."
Engineer: "Congratulations! You've got your vision back! That will be $500."

Had he come back again suffering a certain gentleman's disorder, I reckon he'd have paid the $500 a couple of times over just to avoid the cure.