Paris-The proposed accord was just hours from a final vote when the glitch was spotted. Someone had changed a single word in the draft text—from a "should" to a "shall"-and suddenly the entire climate deal appeared at risk of faltering.Secretary of State John F. Kerry phoned his old friend, French Foreign Minister Laurent Fabius,in exasperation over a tiny revision that implied huge new legal and financial obligations."We cannot do this," Kerry warned.Less than four hours later,the accord was approved with the bang of a gavel. A bit of diplomatic finesse had excised the troublesome word and helped clinch a historic...