A Reddit thread asks if guest posting is safe and asks how you can ensure the site posting your guest blog post uses a dofollow. John Mueller from Google spotted the thread and responded with a joke that read "just make sure not to let Google know."
I suspect the joke was intentional - i.e. if you do not let Google know about the guest blog post (it is not indexed or nofollowed) then it will be fine. Not that you should try to hide your guest blog posting activity and questions in a public forum...
Or maybe I am looking too much into John's response here. Maybe it was a threat and not a joke? But I suspect it was more of a joke...
And we all can use SEO jokes, right?
Here is a screenshot of it:
Forum discussion at Reddit.