Wherever it goes in the film, I agree with 005 that it should be in the credits. First and second (maybe even third?) listings.
I don’t think it really fits in the credits.
I agree. No matter what, it’s going to be a bit of an OT insider joke. But at least if it’s on a screen somewhere, or similar, it can be recognized as an error-type message by others. Putting it in the credits or crawl really only makes sense to those of us reading the thread. And even then, I would argue that the text’s presence alone isn’t very funny (see HansiG’s post), especially once you’ve seen a few Markdown jokes already.