“There’s nothing more dangerous than a good idea.”

Source: The Autobiography of Calvin Coolidge (1929)

Source: The 48 Laws of Power

Source: The Wordy Shipmates

Prayers For The Assassin (2006)

As quoted in Design for Power : The Struggle for the World (1941) by Frederick Lewis Schuman, p. 200; This is the earliest citation yet found for this or similar statements which have been attributed to David Lloyd George, as well as to Benjamin Disraeli, Winston Churchill, Vaclav Havel, Jeffrey Sachs, Rashi Fein, Walter Bagehot and Philip Noel-Baker. It has been described as a Greek, African, Chinese, Russian and American proverb, and as "an old Chassidic injunction". Variants:
Don't be afraid to take a big step if one is indicated. You can't cross a chasm in two small jumps.
The most dangerous thing in the world is to try to leap a chasm in two jumps.
Later life

The Bonhoeffer Reader https://www.google.com/books/edition/The_Bonhoeffer_Reader/CNZgAwAAQBAJ?hl=en&gbpv=1&pg=PA766, p. 766
Letters and Papers from Prison (1967; 1997), On Stupidity

Part 4, Section 7
A Treatise of Human Nature (1739-40), Book 1: Of the understanding
Context: This deficiency in our ideas is not, indeed, perceived in common life, nor are we sensible, that in the most usual conjunctions of cause and effect we are as ignorant of the ultimate principle, which binds them together, as in the most unusual and extraordinary. But this proceeds merely from an illusion of the imagination; and the question is, how far we ought to yield to these illusions. This question is very difficult, and reduces us to a very dangerous dilemma, whichever way we answer it. For if we assent to every trivial suggestion of the fancy; beside that these suggestions are often contrary to each other; they lead us into such errors, absurdities, and obscurities, that we must at last become asham'd of our credulity. Nothing is more dangerous to reason than the flights of the imagination, and nothing has been the occasion of more mistakes among philosophers. Men of bright fancies may in this respect be compar'd to those angels, whom the scripture represents as covering their eyes with their wings. This has already appear'd in so many instances, that we may spare ourselves the trouble of enlarging upon it any farther.

Source: 1960s, Strength to Love (1963), Ch. 4 : Love in action, Sct. 3