Peter Cook's wife, Wendy was nine months pregnant when filming began. Director Bryan Forbes promised them that he would let Peter leave the set as soon as Wendy went into labor. He kept his word, and Peter made it to the hospital just in time for the birth of his daughter, Daisy. Forbes, Dudley Moore, Michael Caine, and Peter Sellers filled his dressing room with flowers and champagne, in celebration of Daisy's birth, when he returned to work.
The film makes reference to a "tontine". This is defined as, according to Wikipedia, "an investment plan for raising capital, devised in the 17th century and relatively widespread in the 18th and 19th centuries. It combines features of a group annuity and a lottery. Each subscriber pays an agreed sum into the fund, and thereafter receives an annuity. As members die, their shares devolve to the other participants, and so the value of each annuity increases. On the death of the last member, the scheme is wound up. In a variant, which has provided the plot device for most fictional versions, upon the death of the penultimate member the capital passes to the last survivor". Historically, most tontines (though not the one described in the film) expired after a fixed time period, upon which the principal was disbursed to those subscribers who had not died.
When Ralph Richardson was offered the part of Joseph Finsbury, he was finishing work on Doctor Zhivago (1965). Richardson wrote to Bryan Forbes from the location in Spain, and asked if he could wear the same jacket he had worn as Alexander in Dr. Zhivago. Forbes agreed, and Richardson did so.
The source novel "The Wrong Box" (1889) was the first of three novels that Robert Louis Stevenson co-wrote with his step-son Lloyd Osbourne. The subsequent books were "The Wrecker" (1892) and "The Ebb-Tide" (1894).