It means you're not supposed to lift an EXACT scenario from another thread and invite people to comment/judge.
If something on a thread inspires you to start a related discussion, and it is likely to result in criticism of the OP on that thread, it would be polite to at the very least wait a few days, and also to spend a few moments thinking about why you are so moved to post and post about your own thoughts, rather than lazily piggybacking off another thread when it's highly likely the OP will read yours.
Just to take a common theme. Let's say a poster made an offhand comment halfway down a thread about her relationship or whatever, and she commented about parent and child parking. You see this, feel annoyed and feel compelled to write a post stating the same EXACT scenario and invite people to respond. The OP later sees this and feels crap because she recognises that it's directed at her, not at P+C parking generally and feels every negative response as a personal pile on. She may have been under some extreme stress at the time, or whatever.
If instead you had thought about it, and written a general post about people misusing P+C parking, she probably won't notice the thread and no harm done and you'll have a lovely boring discussion that goes on for 100 pages about parking.
If you'd waited until the next day or two, and it was still niggling at you so you decided to post, the chances are she might not even be online and wouldn't see it at all.