And I have again observed, my dear friend, in this trifling affair, that misunderstandings and neglect occasion more mischief in the world than even malice and wickedness. At all events, the two latter are of less frequent occurrence.

[Und ich habe, mein Lieber, wieder bei diesem kleinen Geschäft gefunden, dass Missverständnisse und Trägheit vielleicht mehr Irrungen in der Welt machen als List und Bosheit. Wenigstens sind die beiden letzteren gewiss seltener.]

Johann Wolfgang von Goethe (1749-1832) German poet, statesman, scientist

Die Leiden des jungen Werthers [The Sorrows of Young Werther], “Letter from May 4th” (1774) [tr. Taylor]
    (Source)

(Source (German)). Other translation:

Misunderstandings and neglect create more confusion in this world than trickery and malice. At any rate, the last two are certainly much less frequent.
[tr. Mayor / Brogan]

Source of the (heavily paraphrased) "Why look for conspiracy when stupidity can explain so much?"

See also Hanlon's Razor.