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Maybe you're referring to "aCropalypse". Also there was an issue once where sections with overpainted solid black color still retained the information in the alpha channel.

https://www.wired.com/story/acropalyse-google-markup-windows...

https://www.lifewire.com/acropalypse-vulnerability-shows-why...

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I also recall at one point some image file format that ended up leaking sensitive info, because it had a embedded preview or compressed image, and the editing program failed to regenerate the preview after a censor attempt.

Was a loooong time ago, so I don’t remember the details.

AT&T leaked information, as did the US Attorney's Office, when they released PDFs with redacted information. To redact, they changed the background of the text to match the color of the text. You could still copy and paste the text block to reveal the original contents.

https://www.cnet.com/tech/tech-industry/at-38t-leaks-sensiti...

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