Did Neil Armstrong muff his lines?
Well, yes and no.
The exact words (and especially, whether that little "a" got left out, or not) have been the subject of much debate. At various times, both Armstrong and NASA have issued self-contradictory statements.
In one version, Armstrong inadvertently left out the "a", in another, the particle was lost in static. I've listened to it repeatedly (ever since I heard it live, as a child), and I've not been able to make up my mind. I don't think there's static, but the sound quality is pretty iffy, so....your guess is as good as mine.
Of course, conspiracy nuts would say that the whole thing was recorded in a studio, anyway....