“All interesting issues in natural history are questions of relative frequency, not single examples. Everything happens once amidst the richness of nature. But when an unanticipated phenomenon occurs again and again—finally turning into an expectation—then theories are overturned.”

Source: The Alchemist (1988), p. 184; this also has been quoted as "What happens once will never happen again. But what happens twice will surely happen a third time."

Wheeler quoted this saying in Complexity, Entropy, and the Physics of Information (1990), p. 10 http://books.google.com/books?id=mdjsOeTgatsC&lpg=PP1&pg=PA10#v=onepage&q&f=false, with a footnote attributing it to "graffiti in the men's room of the Pecan Street Cafe, Austin, Texas". Later publications, such as Paul Davies' 1995 book About Time (p. 236), credited Wheeler with variations of this saying, but the quip is actually much older. The earliest known source is Ray Cummings' 1922 science fiction novel The Girl in the Golden Atom, http://www.gutenberg.org/ebooks/21094 Ch. V: " 'Time,' he said, 'is what keeps everything from happening at once.' " It also appears in his 1929 novel The Man Who Mastered Time. http://books.google.com/books?id=YdZEAAAAYAAJ&q=%22everything+from+happening+at+once%22#search_anchor The earliest known occurrence other than Cummings is from 1962 in Film Facts: Volume 5, p. 48 http://books.google.com/books?id=sr0vAQAAIAAJ&q=%22everything+from+happening+at+once%22.
Misattributed

""Mais"" is French for ""but"" and the ""but in my opinion"" comes from the context of the original conversation. apud me omnia fiunt Mathematicè in Natura is in latin.
Sometimes the Latin version is incorrectly quoted as Omnia apud me mathematica fiunt.
Sources: Correspondence with Mersenne http://fr.wikisource.org/wiki/Page%3aDescartes_-_%C5%92uvres,_%C3%A9d._Adam_et_Tannery,_III.djvu/48 note for line 7 (1640), page 36, Die Wiener Zeit http://books.google.com/books?id=9Xh3fVZLCycC&pg=PA532&lpg=PA532&dq=%22Omnia+apud+me+mathematica+fiunt%22+original+zitat&source=bl&ots=CgQOrveRiM&sig=WFHwIK20r5vRZ66FwCaxo857LCU&hl=de&sa=X&ei=_Wf2UcHlJYbfsgaf1IHABg#v=onepage&q=%22Omnia%20apud%20me%20mathematica%20fiunt%22%20original%20zitat&f=false page 532 (2008); StackExchange Math Q/A Where did Descartes write... http://math.stackexchange.com/questions/454599/where-did-descartes-write-with-me-everything-turns-into-mathematics?noredirect=1#comment978229_454599

It seems that this quote has only begun to be attributed to Einstein recently, the earliest published source located being the 2008 book Visualization for Dummies by Bernard Golden, p. 85 http://books.google.com/books?id=2ppZkdmpSlgC&lpg=PP1&pg=PA85#v=onepage&q&f=false. Before that it was often attributed to the physicist John Wheeler, who quoted the saying in Complexity, Entropy, and the Physics of Information, p. 10 http://books.google.com/books?id=mdjsOeTgatsC&lpg=PP1&pg=PA10#v=onepage&q&f=false. In fact, this quip is much older; the earliest source located is Ray Cummings' 1921 short story "The Time Professor", which includes the passage https://books.google.com/books?id=sXpDAQAAMAAJ&pg=PA371#v=onepage&q=%22time%20is%20what%20keeps%20everything%20from%20happening%20at%20once%22&f=false: '"I do know what time is," Tubby declared. He paused. "Time," he added slowly -- "time is what keeps everything from happening at once ...".' Cummings repeated the quote in his 1922 science fiction novel The Girl in the Golden Atom, available on Project Gutenberg here http://www.gutenberg.org/ebooks/21094 (according to Science-Fiction: The Early Years by Everett F. Bleiler, p. 171 http://books.google.com/books?id=KEZxhkG5eikC&lpg=PP1&pg=PA171#v=onepage&q&f=false, the novel was a composite of two earlier stories published in 1919 and 1920). Chapter V http://www.gutenberg.org/files/21094/21094-h/21094-h.htm#CHAPTER_V contains the following paragraph: The Big Business Man smiled. "Time," he said, "is what keeps everything from happening at once." The next-earliest source found for this quote is another book by Ray Cummings, The Man Who Mastered Time http://books.google.com/books?id=YdZEAAAAYAAJ&q=%22everything+from+happening+at+once%22#search_anchor from 1929, and no published examples of the quote from authors other than Cummings can be found until the 1962 Film Facts: Volume 5 where it appears on p. 48 http://books.google.com/books?id=sr0vAQAAIAAJ&q=%22everything+from+happening+at+once%22#search_anchor. So, it seems likely that Ray Cummings is the real originator of this saying.
Misattributed

Source: Gregor and the Curse of the Warmbloods

Open letter to Dr. Gustáv Husák, Communist President (8 April 1975)
Context: Life cannot be destroyed for good, neither … can history be brought entirely to a halt. A secret streamlet trickles on beneath the heavy lid of inertia and pseudo-events, slowly and inconspicuously undercutting it. It may be a long process, but one day it must happen: the lid will no longer hold and will start to crack. This is the moment when something once more begins visibly to happen, something truly new and unique … something truly historical, in the sense that history again demands to be heard.

The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next (2007)

Source: The New Left: The Anti-Industrial Revolution (1971), p. 136