I’m watching Boogie Nights and there’s that montage near the end when everything just starts to fall apart for all of the characters. There’s this evil-sounding ominous music just droning in the background and it goes through scenes of all the different characters “hitting bottom” - no point in spoiling it but I assume most people have seen this movie and know the part I’m talking about.
The other, stylistically similar montage I can think of is in Requiem For A Dream.
What are some others?
Revenge of the Sith has a montage of various Jedi falling victim to the enactment of Order 66.
**Almost Famous ** had a pretty good “everything is falling apart” theme towards the end.
Oh, and Fight Club. Everything/Anything was going out of control towards the end of that.
The Dawn of the Dead re-make had a great montage/prologue, set to some really dark Johny Cash.
Casino fits the bill, with all the Las Vegas mafia guys getting whacked.
Yeah, but Scorsese rips it off from his earlier (and superior) Goodfellas. The entire “May 11, 1980” scene is a virtuoso bit of editing (kudos to Thelma Schoonmaker) is an awesome portrayal of the protagonist completely coming apart as he tries to balance his drug deal, wife and mistress, and cooking for a family gathering all in one day. Cidade de Deus also has a montage (where Knockout Ned joints with Carrot to fight L’il Ze) of things progressively getting worse. The Candidate has a scene near the end where Redford’s character, Bill McKay, is getting so wound up in the campaigning process that he completely forgets who he is or what he is trying to accomplish; he ends up gibbering in the back seat of a limo, and trying to film a political announcement at a t.v. studio where he keeps cracking up in laughter.
Stranger
I also wanted to throw in American Beaty, everything went to hell in a handbasket at the end of that one. (Still rates #1 on my all-time favorite movie list though.) 
Damned, this guest membership of mine, runs out June 30th, guess I’ll have to pony up the $$. 
Welcome aboard!
I’m trying to think of a specific example in anime, for the heck of it, and discovering that I can’t mention one. Every one that should be there tends not to be shown, but discovered in partial flashback later.
Not sure if it counts, but I’ve always loved the sequence in 28 Days Later showing the empty London with Godspeed You! Black Emporer playing in the background.
Another one that might not count, the scene in The Long Good Friday, where Hoskins character has just been abducted and in the back seat of his car, thinks over all the moments in the film where he made the wrong choice.
Hoskins didn’t think it would work when he was doing it, but was pleased with the result. It’s on youtube from 1:48 onwards.
The bit in The Usual Suspects when the cop puts the pieces together.
In last years big gangster movie American Gangster when they bust Denzel. I thought it was quite well done with all the action taking place while he is in church with his wife and mother.
(using spoiler brackets since it came out last year, and at least I hadn’t seen it untill this week)
Willard looking haunted while the bombs fall and The Doors’ “This is the End,” IIRC, plays at the end of Apocalypse Now.
The characters meeting their various sad fates at the end of On the Beach.
Would the montage of all of Michael Corleone’s enemies getting whacked during the baptism at the end of The Godfather count?
Good Morning Vietnam - set to What a Wonderful World
Rocky IV (Well from Ivan Drago’s point of view it’s falling apart ;).)
The whole end of Dr. Strangelove, showing all the nukes going off.
No, that’s the exact opposite: a plan coming together.
But destroying Michael Corleone’s soul in the process, hence the ironic declarations that he rejects the devil even as he’s finally embracing him.