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 through a domino effect.
Poet George Herbert included it in a 1640 collection of aphorisms as "For want of a naile the shoe is lost, for want of a shoe the horse is lost, for want of a horse the rider is lost." Benjamin Franklin, in his 1758 Poor Richard's Almanack, recorded a longer version:

History

The proverb is found in a number of forms.
Predecessors include the following:

Middle High German : 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.
For sparinge of a litel cost, Fulofte time a man hath lost, The large cote for the hod.
Middle French: Par ung seul clou perd on ung bon cheval. .
"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".