The elderly servant handed a card, with a number inscribed on it, to the new visitor, said in a whisper, "Your number will be called, sir, in your turn," and disappeared.

For some minutes nothing disturbed the deep silence but the faint ticking of a clock.

After a while a bell rang from an inner room, a door opened, and a gentleman appeared, whose interview with Doctor Lagarde had terminated.
His opinion of the sitting was openly expressed in one emphatic word--"Humbug!" No contribution dropped from his hand as he passed the money-box on his way out.
The next number (being Number Fifteen) was called by the elderly servant, and the first incident occurred in the strange series of events destined to happen in the Doctor's house that night.
One after another the three men who had been waiting rose, examined their cards under the light of the lamp, and sat down again surprised and disappointed.
The servant advanced to investigate the matter.

The numbers possessed by the three visitors, instead of being Fifteen, Sixteen and Seventeen, proved to be Sixteen, Seventeen and Eighteen.

Turning to the stranger who had arrived the last, the servant said: "Have I made a mistake, sir?
Have I given you Number Fifteen instead of Number Eighteen ?" The gentleman produced his numbered card.
A mistake had certainly been made, but not the mistake that the servant supposed.