In A Few Good Men, Jack Nicholson’s character never utters the first part of the quote. He actually says, “You want answers?” Then Tom Cruise says, “I think I’m entitled to them!” Nicholson asks again, “You want answers?” To which Cruise replies, “I want the truth!” It’s only then that Nicholson says, “You can’t handle the truth!”
"That's
one
small
step
for man,"
...and then changes in the second part when he realizes he missed the "a"...
"one
giant leap
for mankind."
It doesn't matter what he meant to say, just what he did say. If he had tripped and said, "Fuck!", that would have been the first word spoken by somebody standing on the moon.