From the Schneier on Security blog
Slate magazine was able to cleverly read the Ghislaine Maxwell deposition and reverse-engineer many of the redacted names.
We’ve long known that redacting is hard in the modern age, but most of the failures to date have been a result of not realizing that covering digital text with a black bar doesn’t always remove the text from the underlying digital file. As far as I know, this reverse-engineering technique is new.
EDITED TO ADD: A similar technique was used in 1991 to recover the Dead Sea Scrolls.