There's an internet "meme" that's been floating around since 2008 which takes what some apparently believe to be a sharp shot to the ribs of Martin Scorsese's The Departed, specifically the shot at the end of the rat skittering across the railing. The meme's joke comes from an episode of The Simpsons that I haven't seen, and it goes like this: "The rat symbolizes obviousness!".
I remember when Scorsese's film came out, and that particular moment was frequently singled out by critics as being a bit of symbolism that was beneath the filmmaker, and as such it made them roll their eyes, gnash their teeth, rend their garments. The problem is that the actual physical rat is no more or less symbolic of Colin Sullivan (Matt Damon) than the repeated use of the word "rat" to refer to the kind of person who poses as a member of one group as a means of hurting or destroying that group. "Rat" is in the lexicon with that definition, and actual rats are conjured in our minds every time we use or hear the word in that context, because words have a tendency to do that.
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So, when people saw the rat scurrying by at the end of The Departed and said "Oh, I know what that means instantly, without even having to think about it! How dumb!" they were actually sort of missing the point a little bit. Because Scorsese wasn't trying to be "symbolic", as we often think of the word, which is to say he wasn't using coded imagery to depict a nebulous idea that would otherwise resist physical representation. No, what Scorsese was doing was this: he was making a joke.
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A dark joke, one that, in effect, takes a swift kick at Sullivan's corpse. It's an insult, a mockery of this awful human being: even in death, we'll never forget what you are, and you're still nothing. The guy's just been shot in the face, and the world is still giving him shit. That's what the rat on the rail is. It's not a rat that symbolizes a metaphorical rat; it's a rat that's laughing at Colin Sullivan's body. Nasty, dark, brutal -- in other words, completely in tone with the rest of the film.
