Never forget how the USA did the exact thing the attackers wanted: went on a needless, hubristic, expensive orgy of war and violence.
Never forget how we destroyed a country that wasn't even involved in the attacks.
Never forget that the country the attackers were from, Saudi Arabia, suffered no consequences whatsoever and was enriched substantially.
Never forget the ridiculous, blatant lies espoused by the US government, and how the entire mainstream press became stenographers of those lies, rather than challenge them.
Never forget how the US began its serious turn into a police surveillance state.
Never forget the open racism and religious bigotry.
Never forget how even the mildest criticism of the entire response and enterprise were met with withering contempt.
Never forget the innocent people tortured at Guantanamo, most of whom are still there over two decades later, and will probably die in illegal US detention.
Never forget the CIA covering up the extent of their torture.
Never forget the excuses and justification made for torture by elected nofficials and party apparatus.
Never forget the 1984-esque nuspeak, like "enhanced interrogation."
Never forget the CIA infiltrating organizations, pushing them into conspiracies they probably wouldn't have considered on their own, then prosecuting them as terrorists for it.
Never forget that 3,000 deaths were plenty of justification to spend trillions of dollars killing people, but over a million deaths isn't enough to wear a fucking mask.
Never forget.