Edit: Though my examples below are all fiction, that's half or less of my current reading list. I also have philosophy, research papers, long-form academic books, popular science books, and the like.
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For reference, here's the next new card for one of the books I'm reading:
Bleak House
Charles Dickens
CHAPTER III
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coach gave me a terrible start.
It said, "What the de-vil are you crying for?"
I was so frightened that I lost my voice and could only answer in a
whisper, "Me, sir?" For of course I knew it must have been the
gentleman in the quantity of wrappings, though he was still looking
out of his window.
"Yes, you," he said, turning round.
"I didn't know I was crying, sir," I faltered.
"But you are!" said the gentleman. "Look here!" He came quite
opposite to me from the other corner of the coach, brushed one of his
[...] furry cuffs across my eyes (but without hurting me), and showed
me that it was wet.
"There! Now you know you are," he said. "Don't you?"
"Yes, sir," I said.
"And what are you crying for?" said the gentleman, "Don't you want to
go there?"
(The prior cards include the text up to and including the line starting "But you are!")
My response, having never seen the passage before, was "coat's"; the correct word is "large". The substitution doesn't meaningfully alter the passage: The only thing we know about the gentleman at this point is that he's a stranger whom Esther has just met in the coach, wearing a large coat (cloak?). This is the first time he speaks in the book.
I selected "again" because this is a new card; if this had been a review, I might have deemed it acceptable. The bar for similarity varies depending on the book the passage came from, my mood on any particular day, and how much I care about the contents of the passage. The Rime of the Ancient Mariner, being poetry, doesn't get any latitude: I must remember the word exactly. Some of the tedious battle descriptions in Le Morte d'Arthur get a pass no matter how badly they go wrong.
If a card is causing me trouble, there are a few corrective actions I can take: In some instances I'll edit the card to blank out a more meaningful word than the one randomly selected, which is an easier and more engaging review task. If I can't figure out the blanked word from context, I'll look it up in a dictionary; this happens most often with my second-language texts. And in extreme cases, I have enough coverage of each book that permanently dropping card or two won't cause problems.
The idea for this came from the paper[1] that introduced the concept of Cloze deletions. They were originally envisioned as a readability score. Instead of calculating statistics about the words used, like traditional methods, Taylor proposed to blank out every Nth word and measure how many could be guessed correctly by a reader. Readability and comprehension are two sides of the same coin. They both rely on observing the relationship between text and reader. If used to evaluate the text, this is called readability; if to evaluate the reader, comprehension.
[1] https://www.gwern.net/docs/psychology/writing/1953-taylor.pd...