“Shakspeare never has six lines together without a fault. Perhaps you may find seven: but this does not refute my general assertion.”
October 19, 1769, p. 170
Life of Samuel Johnson (1791), Vol II
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Samuel Johnson 362
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Barack Obama: "Remarks at the Department of State," May 19, 2011. Online by Gerhard Peters and John T. Woolley, The American Presidency Project. http://www.presidency.ucsb.edu/ws/index.php?pid=90397&st=&st1=
2011
“Perhaps I’m tired of waiting for something I may never find.”
Source: Sweet Persuasion

Letter to Rev. C. B. Tayler ( 8 July 1853) in Ch. 6 : Undergraduate Life At Cambridge October 1850 to January 1854 — ÆT. 19-22, p. 189
The Life of James Clerk Maxwell (1882)
Context: I maintain that all the evil influences that I can trace have been internal and not external, you know what I mean—that I have the capacity of being more wicked than any example that man could set me, and that if I escape, it is only by God's grace helping me to get rid of myself, partially in science, more completely in society, — but not perfectly except by committing myself to God as the instrument of His will, not doubtfully, but in the certain hope that that Will will be plain enough at the proper time. Nevertheless, you see things from the outside directly, and I only by reflexion, so I hope that you will not tell me you have little fault to find with me, without finding that little and communicating it.

“Never find fault with the absent.”
Absenti nemo non nocuisse velit.
Sextus Propertius, Elegies, II, xix, 32, also translated: "Let no one be willing to speak ill of the absent".
Misattributed

Source: Contributions to Analytical Psychology (1928), p. 193

Qu'on me donne six lignes écrites de la main du plus honnête homme, j'y trouverai de quoi le faire pendre.
As quoted in The Cyclopedia of Practical Quotations (1896) by Jehiel K̀eeler Hoyt, p. 763
Édouard Fournier, in L'Espirit dans l'Historie (1867), 3rd edition, Ch. 51, p. 260, disputes the traditional attribution, and suggests various agents of Richelieu might have been the actual author.
David Hackett Fischer, in Champlain's Dream (2009), Simon & Schuster, p. 704, n. 14, says it's a paraphrase of Quintilian and there is no source closer to Richelieu than Francoise Bertaut's Memoires pour servir à l'histoire d'Anne d'Autriche.
Disputed

“To find fault is easy; to do better may be difficult.”

12 May 1830
Table Talk (1821–1834)