A dictionary of Proverbs (Oxford Paperback Reference)

For WANT of a nail the shoe was lost; for want of a shoe the horse was lost; and for want of a horse the man was lost
The proverb is found in a number of forms. Cf. late 15th-cent. Fr. par ung seul clou perd on ung bon cheval, by just one nail one loses a good horse; c 1390 GOWER Confessio Amantis v. 4785 For sparinge of a litel cost Fulofte time a man hath lost The large cote for the hod [hood].

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  1629 T. ADAMS Works 714 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. 1640 G. HERBERT Outlandish Proverbs no. 499 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. 1880 S. SMILES Duty x. ‘Don’t care’ was the man who was to blame for the well-known catastrophe:—’For want of a nail the shoe was lost, for want of a shoe the horse was lost, and for want of a horse the man was lost.’ 1925 S. O’CASEY Juno & Paycock 1. 16 You bring your long-tailed shovel, an’ I’ll bring me navvy [device for excavating earth]. We mighten’ want them, an’, then agen, we might: for want of a nail the shoe was lost, for want of a shoe the horse was lost, an’ for want of a horse the man was lost—aw, that’s a darlin’ proverb, a daarlin’. 1979 M. McCARTHY Missionaries & Cannibals viii. No detail.. was too small to be passed over. .. ‘For want of a nail,’ as the proverb said. 1995 National Review 12 June 10 For want of nail the battle was lost. Well, Republicans may have found just the right nail to win the entitlement battle. “action and consequence; great and small
  

If you WANT something done, ask a busy person
The rationale behind this apparent paradox is indicated in quot. 1997.

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  1984 Christian Science Monitor 26 Oct. 38 It means. specifically, that you must banish all idleness; and it also means, in a general way, that if you want something done, you should ask a busy person—like me—to do it. 1997 Life Association News Aug. 60 It’s the old story: If you want something done, ask a busy person. They know how to manage their time. 1998 Times 9 Jan. 33 They do say that if you want something done you should ask a busy person, but there must be limits. “efficiency and inefficiency
  

want see also (noun) WILFUL waste makes woeful want; (verb) the MORE you get, the more you want; if you want PEACE, you must prepare for war; WASTE not, want not.