Paying The Doctor
Before the introduction of the National Health Service in England, it was sometimes considered unlucky to pay a doctor’s bill in full. To do so implied a dangerous confidence in restored and continuing health that could only bring misfortune, and would probably result in his services being needed again very soon. Some patients, therefore, made it a rule to hold back a token sum, perhaps a shilling, from the total amount due.
On the other hand, gypsies always paid their bills punctually and in full, because they believed that unpaid for medicine would not work.
Source: The Encyclopedia of Superstitions