I started making comments like this after the tenth time seeing a green account make a joke, get downvoted to hell and reply "WTF! Why is Hacker News so mean to noobs!" (Peter was green an hour ago)
That's the problem when forums have a lot of sarcasm. It becomes even more difficult for anyone to judge your intentions. When people in "witty" forums say mean/dumb things they get surprised by how many readers think they are serious. And, some of them respond with mean/dumb things in seriousness. And, in not-seriousness... Usually there's an indistinguishable mix. And, even when you say something constructive, a lot of people will assume a negative intention and get upset that you are so disparaging.
So, yeah. Every few months I get in the mood to welcome a witty noob. Doesn't always go well :P
After all, who is to know that a "throwaway" account actually gets thrown away?