This is in the Fatine section of the book.
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1) Fantine falls for a young man not of her station. She makers him her first lover and becomes pregnant.
2) Fantine leaves her daughter with a family she believes will raise her in a happy home alongside their own daughters. This couple extorts any money she can earn.
3) Fantine has her letters to the couple, the Thénardiers, scribed because she cannot read or write. The scribe gossips about her, and she loses her job because she's had an illegitimate child.
Note- None of these things would classify as unforgivable. She did the best she could under the circumstances and believed everything she did was the best for her child.