For Want of a Nail

"For want of a nail" is a proverb, having numerous variations over several centuries, reminding that seemingly unimportant acts or omissions can have grave and unforeseen consequences.

History
The proverb is found in a number of forms. Benjamin Franklin included a version in his Poor Richard's Almanack (1758), but over a century earlier, the poet George Herbert included it in a 1640 collection of aphorisms. Predecessors include the following:
 * Middle High German (positively formulated): Diz ſagent uns die wîſen, ein nagel behalt ein îſen, ein îſen ein ros, ein ros ein man, ein man ein burc, der ſtrîten kan. ("The wise tell us that a nail keeps a shoe, a shoe a horse, a horse a man, a man a castle, that can fight.") (c. 1230 Freidank Bescheidenheit)
 * "For sparinge of a litel cost, Fulofte time a man hath lost, The large cote for the hod." ("For sparing a little cost often a man has lost the large coat for the hood.") (c 1390 John Gower, Confessio Amantis v. 4785–4787)
 * Middle French: "Par ung seul clou perd on ung bon cheval." (Modern French: "Par seulement un clou, on perd un bon cheval."; English: "By just one nail one loses a good horse.") (c 1507 Jean Molinet, Faictz Dictz D., v768).
 * "The French-men haue a military prouerbe; 'The losse of a nayle, the losse of an army'. The want of a nayle looseth the shooe, the losse of shooe troubles the horse, the horse indangereth the rider, the rider breaking his ranke molests the company, so farre as to hazard the whole Army". (1629 Thomas Adams (clergyman), "The Works of Thomas Adams: The Sum of His Sermons, Meditations, And Other Divine And Moral Discourses", p. 714")