Nothing is more terrible than ignorance in action.

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

Sprüche in Prosa: Maximen und Reflexionen [Proverbs in Prose: Maxims and Reflections] (1833) [tr. Saunders (1893)]
    (Source)

Goethe's use of this phrase comes up twice in the Maxims and Reflections. Other translations:

From Art and Antiquity, Vol. 5, #3, Individual Points (1826):

Es ist nichts schrecklicher als eine tätige Unwissenheit.
(Source (German))

Nothing is more terrible than ignorance in action.
[tr. Saunders (1893), "Life and Character," sec. 3, #231]

There is nothing more frightful than ignorance in action.
[tr. Rönnfeldt (1900)]

There is nothing more dreadful than active ignorance.
[tr. Stopp (1995), #367]

From Wilhelm Meister's Journeyman Years (1829):

Auch nichts schrecklicher ist, als die Unwissenheit handeln zu sehen.
(Source(German))

There is no more terrible sight than ignorance in action.
[tr. Saunders (1893), "Life and Character," sec. 1, #52]

There is nothing more terrible than to see ignorance in action.
[tr. Rönnfeldt (1900)]

Nothing is more frightful than to see ignorance in action.
[tr. Stopp (1995), #367]