If you haven't seen Once Upon a Time in America then you may want to skip these brief lines. I'm not discussing a serious narrative spoiler, but those who prefer carte blanche should look away.
When Dominic meets his fate he tells Noodles: "I slipped." These words, which appear to quietly haunt Noodles for the rest of his life, blindside us. Like an iceberg set upon our Titanic, they come unbidden to catch us unawares—extraneous to plot development and not even immediately applicable to pathos, "I slipped" is like a perpendicular insertion into the linear progression of time and the narrative. Of course, Once Upon... is not a linear film and its conceptualization of time, memory, history, and diegetic reality is like a Möbius strip (and in this it has a common overarching feature with another great philosophical genre film of the time, Videodrome). There are many such perpendicular insertions, touches which seem to come from out of "nowhere," but which make prevent any such sleek gangster movie. This movie spreads outward, and more on that aspect in the future. But like icebergs and Titanics, the iceberg was always there first, and our own shortcomings of perception and planning, our habitual shackles, are truly to blame: the iceberg may appear out of the mist but it does not come to be out of the mist. So when Dominic says, "I slipped," we get a split second that's easy to accept, but hard to assimilate. For a moment the narrative line morphs into a sturdy horizontal cross-section of these big concepts, "America," "youth," "masculinity," "violence," etc. Almost all such narrative treatments of these kinds of Big Issues in film appear clumsy, shallow, obsequious next to Leone's film.
Time means something different for this moment; not a narrative time but an intrusion, a pause on narrative chronology to reflect upon the underlying experience that burns off the moment we comprehend a narrative through-line. What kind of cross-section here? Actually Once Upon a Time in America is not at all a cross-section of America, or of children or men, or of American Jews or New Yorkers. Its handling of all these things tends toward the narrow and specific, the personal, and if these are ever elevated to generalized principles (anecdotal, nostalgic, exemplary) it is only because the children's narrative, at the very least, is the one sure aspect of the story organized under the sign of memory. When one looks back one has to come up with ways to make sense of the fragmented and stylized slivers that comprise our private, experiential histories.
Hence "I slipped" means a lot of things. It tells us something about the miserable and admirable courage of these hustling street kids; Dominic, too proud to admit he got shot, but too close to death to be proud consciously, has to blurt out something that comes to mind—anything that might save face in front of his pals. It's the sort of explanation that comes to a sleepwalker's mind when she has been awakened; those of us who have been in this position ourselves will understand the weird explanations (neither lies, nor false, evasive but naked) that come to the lips, and we may fancy a guess that this is the sort of last hurrah of Dominic's experiential self we see. Of course: he slipped.