"She was sitting in an office with other parents and their children when one of the mothers 'accidentally' spilled coffee on Grimes’s dress just before she went in for her interview."

"'It might have intimidated some little girls, but it gave me something to talk about,' she said. Capra, who 'handpicked every single person in that film,' according to Grimes, hired her." 

From "As ‘It’s A Wonderful Life’ turns 75, Karolyn ‘Zuzu’ Grimes reflects on the film that belatedly changed her life" (WaPo).

Grimes played George Bailey’s youngest daughter, Zuzu, the “little ginger snap” with the petals, who in the film’s profoundly soul-stirring climax, says perhaps the film’s most-quoted line: “Look, Daddy, teacher says every time a bell rings, an angel gets his wings.” 

“I had no clue whatsoever those words would be so special to so many people,” she said. “I’m thrilled I got to say them and that I got to be a part of that scene and that movie.”

Every time you spill coffee on somebody, an angel gets the part you were trying to deny her.