“It is almost as difficult to make a man unlearn his errors as his knowledge. Mal-information is more hopeless than non-information; for error is always more busy than ignorance. Ignorance is a blank sheet, on which we may write; but error is a scribbled one, on which we must first erase. Ignorance is contented to stand still with her back to the truth; but error is more presumptuous, and proceeds in the same direction. Ignorance has no light, but error follows a false one. The consequence is, that error, when she retraces her footsteps, has further to go, before she can arrive at the truth, than ignorance.”

L'ignorance et l'erreur sont nécessaires à la vie comme le pain et l'eau.
Pierre Nozière http://fr.wikisource.org/wiki/Pierre_Nozi%C3%A8re_-_Livre_deuxi%C3%A8me._Notes_%C3%A9crites_par_Pierre_Noziere_en_marge_de_son_gros_Plutarque. (1899), book II: Notes écrites par Pierre Nozière en marge de son gros Plutarque

Vol. I, Preface, p. xiii
Expository Thoughts on the Gospels: St. John (1865–1873)

Dijkstra (1976-79) On the foolishness of "natural language programming" https://www.cs.utexas.edu/users/EWD/transcriptions/EWD06xx/EWD667.html (EWD 667)
1970s

Maxim 715, trans. Stopp
Maxims and Reflections (1833)

"Religion: A Dialogue."
Variant translation: To free a man from error does not mean to take something from him, but to give him something.
Essays
Source: Essays and Aphorisms
Context: To free a man from error is to give, not to take away. Knowledge that a thing is false is a truth. Error always does harm; sooner or later it will bring mischief to the man who harbors it. Then give up deceiving people; confess ignorance of what you don't know, and leave everyone to form his own articles of faith for himself. Perhaps they won't turn out so bad, especially as they'll rub one another's corners down, and mutually rectify mistakes. The existence of many views will at any rate lay a foundation of tolerance. Those who possess knowledge and capacity may betake themselves to the study of philosophy, or even in their own persons carry the history of philosophy a step further.

Of Heresies
Meditationes sacræ (1597)

Es ist so gewiß als wunderbar, daß Wahrheit und Irrthum aus Einer Quelle entstehen; deßwegen man oft dem Irrthum nicht schaden darf, weil man zugleich der Wahrheit schadet.
Maxims and Reflections (1833)

Source: 1920s, Prejudices, Third Series (1922), Ch. 3 "Footnote on Criticism", pp. 85-104
Context: Truth, indeed, is something that is believed in completely only by persons who have never tried personally to pursue it to its fastness and grab it by the tail. It is the adoration of second-rate men — men who always receive it as second-hand. Pedagogues believe in immutable truths and spend their lives trying to determine them and propagate them; the intellectual progress of man consists largely of a concerted effort to block and destroy their enterprise. Nine times out of ten, in the arts as in life, there is actually no truth to be discovered; there is only error to be exposed. In whole departments of human inquiry it seems to me quite unlikely that the truth ever will be discovered. Nevertheless, the rubber-stamp thinking of the world always makes the assumption that the exposure of an error is identical with the discovery of truth — that error and truth are simply opposites. They are nothing of the sort. What the world turns to, when it has been cured of one error, is usually simply another error, and maybe one worse than the first one. This is the whole history of the intellect in brief. The average man of today does not believe in precisely the same imbecilities that the Greek of the Fourth Century before Christ believed in, but the things that he does believe in are often quite as idiotic.
Perhaps this statement is a bit too sweeping. There is, year by year, a gradual accumulation of what may be called, provisionally, truths — there is a slow accretion of ideas that somehow manage to meet all practicable human tests, and so survive. But even so, it is risky to call them absolute truths. All that one may safely say of them is that no one, as yet, has demonstrated that they are errors. Soon or late, if experience teaches us anything, they are likely to succumb too. The profoundest truths of the Middle Ages are now laughed at by schoolboys. The profoundest truths of democracy will be laughed at, a few centuries hence, even by school-teachers.