Quotes about happening
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Source: The Tao of Pooh
Source: Midnight in the Garden of Good and Evil

Source: Hero

“… as long as nothing happens between them, the memory is cursed with what hasn't happened.”
Source: Blue Eyes, Black Hair
“But I should have known that it doesn't take that long for change to happen - it takes a second.”
Source: United We Spy

“We could imagine all sorts of universes unlike this one, but this is the one that happened.”
Source: Extremely Loud and Incredibly Close

According to TruthOrFiction.com https://www.truthorfiction.com/did-dwight-eisenhower-say-someday-someone-will-claim-it-never-happened-in-1945/, this sentence first appeared in a letter to the editor published on DominicanToday.com, accompanied with the words "he did this because he said in words to this effect". It was probably a paraphrase of the above bold sentence.
Disputed

“Everything that happens before Death is what counts.”
Variant: Is Death important? No. Everything that happens before death is what counts.
Source: Something Wicked This Way Comes
“But sometimes normal just isn't happening. Sometimes crazy feels too good to resist.”
Source: Blue-Eyed Devil
“I would not have had that happen to you. Discourtesy is unspeakably ugly to me.”
Source: The Silence of the Lambs

“It sometimes happens that a woman is handsomer at twenty-nine than she was ten years before.”
Source: Pack of Two: The Intricate Bond Between People and Dogs

“Why isn't life better designed so it warns you when terrible things are about to happen?”
Source: After You'd Gone

Source: Northanger Abbey: a play in two acts, based upon the novel
“I'm the guy who happened to be home the night Kat came to steal a Monet."- Hale”
Source: Heist Society

Newspaper interview (1902), when asked what qualities a politician required, Halle, Kay, Irrepressible Churchill. Cleveland: World, 1966. cited in Churchill by Himself (2008), ed. Langworth, PublicAffairs, p. 489 ISBN 1586486381
Early career years (1898–1929)
Source: Bambi vs. Godzilla: On the Nature, Purpose, and Practice of the Movie Business

“Meeting you has been the best thing that's ever happened to me. - John Tyree”
Variant: In a lifetime of mistakes, you two are the greatest things that have ever happened to me.
Source: Dear John
Also in Recipes from an Edwardian Country House: A Stately English Home Shares Its Classic Tastes by Laura Schaefer [Simon & Schuster, 2013, ISBN 1-476-73033-4] ( p. 22 https://books.google.com/books?id=zZPzAQAAQBAJ&pg=PA22)
Source: The Moonstone [Street, 1868] ( p. 49 https://books.google.com/books?id=FmsOAAAAQAAJ&pg=PA49).

By The River Piedra I Sat Down and Wept (1994)
Source: By the River Piedra I Sat Down and Wept
Context: You have to take risks … We will only understand the miracle of life fully when we allow the unexpected to happen. Every day, God gives us the sun — and also one moment in which we have the ability to change everything that makes us unhappy. Every day, we try to pretend that we haven't perceived that moment, that it doesn't exist — that today is the same as yesterday and will be the same as tomorrow. But if people really pay attention to their everyday lives, they will discover that magic moment. It may arrive in the instant when we are doing something mundane, like putting our front-door key in the lock; it may lie hidden in the quiet that follows the lunch hour or in the thousand and one things that all seem the same to us. But that moment exists — a moment when all the power of the stars becomes a part of us and enables us to perform miracles.

“Happens to me all the time… People are always underestimating my dumbness.”
Source: Motor Mouth

“Everyone makes mistakes, even if some are worse than others. Accidents happen.”
Source: The Best of Me
Source: Kiss of a Demon King
Source: Between The Tides

“Don’t aim for perfection. Evolution, and life, only happen through mistakes.”
Source: The Humans

“You’ve got to be kid— Well, crud, what just happened there? I ran out of syl”
Source: The Hidden Oracle