“A little knowledge is a dangerous thing. I regret that this isn't fatal.”

Source: The Poet's Diary (1904), p. 158.

Misattributed

A variation on a quotation of Alexander Pope, attributed to Einstein in various recent sources, such as Marvin Minsky's The Emotion Machine (2006), p. 176 http://books.google.com/books?id=OqbMnWDKIJ4C&lpg=PP1&pg=PA176#v=onepage&q&f=false, and at the start of the 2006 pilot episode of the television series Eureka. The oldest published source located attributing this to Einstein is the 2004 book Strategic Investment: Real Options and Games by Han T. J. Smit and Lenos Trigeorgis, p. 429 http://books.google.com/books?id=pN41ZtNoqBEC&lpg=PP1&pg=PA429#v=onepage&q&f=false, and before that it was attributed to him on the internet, the earliest example found being this post from 19 May 1995 http://groups.google.com/group/bit.listserv.physhare/msg/ef186aec3bf66ba6. But long before that, the same quote appears in an advertisement for Encyclopaedia Britannica that ran in The Atlantic Monthly: Volume 216 from 1965, p. 139 http://books.google.com/books?id=TuMmAQAAIAAJ&q=%22so+is+a+lot%22#search_anchor. The ad mentioned Einstein but did not directly attribute the quote to him: "Encyclopaedia Britannica says: A little knowledge is a dangerous thing. So is a lot. The more you know, the more you need to know — as Albert Einstein, for one, might have told you. Great knowledge has a way of bringing with it great responsibility. The people who put the Encyclopaedia Britannica together feel the same way. After all, if most of the world had come to count on you as the best single source of complete, accurate, up-to-date information on everything, you'd want to be pretty sure you knew what you were talking about."
Misattributed

"On Elementary Instruction in Physiology" (1877) http://aleph0.clarku.edu/huxley/CE3/ElPhys.html
1870s
Context: The saying that a little knowledge is a dangerous thing is, to my mind, a very dangerous adage. If knowledge is real and genuine, I do not believe that it is other than a very valuable possession, however infinitesimal its quantity may be. Indeed, if a little knowledge is dangerous, where is the man who has so much as to be out of danger?

Source: Hunters of the Dusk

Source: Equal Rites

Source: Demon Princes (1964-1981), The Star King (1964), Chapter 8 (p. 83)

Source: The Picture of Dorian Gray

Original French: Je prononce à regret cette fatale vérité... mais Louis doit mourir, parce qu'il faut que la patrie vive.
Speech to the National Convention http://www.royet.org/nea1789-1794/archives/journal_debats/an/1792/convention_1792_12_03.htm on the judgment of Louis XVI (3 December 1792)