That might be because Jamie Zawinski's website, which that link links to, does something ... special ... when it sees HN in the "Referer" field, so anyone actually following that link from here would (unless they take special measures) get IIRC a famous obscene image rather than the actual post you were hoping to link to.
(JWZ is not a fan of Hacker News and its community.)
Ah, OK. My hazy memory was that it was the goatse.cx image, which is for most people more unpalatable than a testicle in an eggcup, but I didn't check exactly what it was :-).
Flagged for what? In this case user - me - was unaware that submitted content gets replaced when requested with Referer header mentioning HN. There should be a 'you cannot post this' warning instead of hidden post creation. I only noticed shadowban by accident: switched to logged-in browser to submit and went back to logged-out for browsing.
Flagged for being NSFW and devoid of interesting content. The average user, like you, is unaware that the content they are seeing is different from the content you submitted.
(JWZ is not a fan of Hacker News and its community.)