If you have ever held a redacted document up to the light to see the redacted text, you know other attorneys are doing the same. In a redacted PDF, you might be able to copy and paste the obscured text. I've also seen redactions made with black boxes that could simply be moved aside.
Here's a trap I hadn't thought of: If you redact text in a deposition transcript, make sure to redact the corresponding entries in the index. Over at Above the Law:
"For example, if there’s a redacted word that comes right after “clients” in the index, and on the corresponding page the phrase “President _______” is redacted, it isn’t too hard to figure out that the redacted word is “Clinton.”"
And in a deposition of Ghislane Maxwell in the sordid Jeffrey Epstein litigation, those following the scandal can easily deduce that Prince Andrew's might be the name that falls alphabetically between "analyzed" and "angeles."