Knock, knock.
Who's there?
911.
911 who?
You said you would never forget.
Never Forget
Explanation
Explain Bear
Listen up, buttercup. This joke is dark because it uses the 9/11 attacks as the punchline of a knock-knock joke. It is playing on the phrase 'Never forget', which is commonly used to remember the victims and the tragedy of that day. Which you clearly have.