Donald Trump managed to mix up his numbers on Monday night, accidentally referring to the 9/11 attacks as taking place on 7/11.
The Republican frontrunner made the mistake in front of thousands of supporters at a rally in New York.
Speaking ahead of a key primary in the state, Trump tried to use the famous 2001 terror attacks to bash his rival Ted Cruz.
He was speaking about “New York values”, a term Cruz used derisively back in January.
"I wrote this out and it's very close to my heart: I was down there and I watched our police and our firemen, down on 7/11, down at the World Trade Center, right after it came down. And I saw the greatest people I’ve ever seen in action."
7/11 is the name of a popular convenience store chain, headquartered in the US.
Trump did not appear to realise he had made the gaffe.
At the start of his rally, around a dozen protesters had interrupted proceedings by sitting on the floor of the arena and locking arms arms.
Police carried them out by their arms and legs, and later confirmed six people were arrested - mainly for disorderly conduct and trespassing, Fox News reported.
Trump is leading by over 30 per cent in opinion polls in the run up to tonight's primary vote.
Trump has won 744 delegates, well ahead of Cruz, who is on 559.
Their Republican rival John Kasich has notched up 144, while Marco Rubio achieved 171 before he dropped out of the race in March.