Obviously, not every piece of dialog written in literature and entertainment is intended to be a catchphrase.
Not every line can be “Go Ahead, Make My Day” or “Et Tu, Brute” or “No Soup for You”
Are there any examples of throwaway lines that became iconic Catch phrases?
I am drawing a blank.
I’ll be back
“That’s no moon…”
I can’t find a cite for this, but I have read that "Where’s the beef?’ was not the intended catchphrase in that Wendy’s commercial. References to a “big, fluffy bun” were supposed to be the memorable lines, but the “Where’s the beef?” part is what passed into the vernacular.
Not sure if that’s what you’re looking for.
ETA" “We’re going to need a bigger boat” was an ad lib in Jaws, I believe.
Dustin Hoffman in Midnight Cowboy – “Hey! I’m walkin’ here!” I read that it was an ad lib – a taxi got too close as he and Voight were crossing the street.
Leave the gun - take the cannoli.
When come back, bring phrase. 
Bart Simpson’s “I didn’t do it”, but only for that one episode.
This one time, at band camp …
Not nearly as iconic as some of the others already mentioned, but I think Max Fischer’s “O.R. they?” quip in Rushmore counts. It’s usually the go- to quote from that movie and was featured prominently in the trailers.
‘You talkin’ to me?’
Maybe this is what you were getting at, but the original was “You speaking to me?” It was in being misquoted in Taxi Driver that it really took off.
“I’ll be in my bunk.”
The problem with this game is, it’s difficult to tell if a line was intended to be iconic or was just a throw-away. Especially after it has achieved iconic status.
I always think of “Four score and seven years ago.”
It was merely an introduction to the main theme of this short speech. It only makes sense because of the timing. Yet, whenever we hear it, we think of HUGE things: Lincoln’s presidency, the American Civil War, slavery, federal government and the question of state’s rights, etc…
Quite an accomplishment for a simple reference to 87 yrs back.
“It’s a trap!”
A couple of examples from pro wrestling:
Arn Anderson made a throwaway reference to himself, Flair, Tully, and Ole as the Four Horsemen of the Apocalypse, and inadvertently named one of the most infamous stables in wrestling history.
Steve Austin made a throwaway line about Austin 3:16 says I just kicked your ass, and it became his catchphrase, got printed on a bunch of t-shirts, etc.
Huh?
I just rewatched the scene to see if I (as the rest of us) have been getting it wrong all these years.
Not sure what version you’re referring to but there’s no doubt it’s “You talking to me?”.
See for yourself here.
KneadToKnow is referring to the fact that Travis was [mis]quoting Shane. It’s unlikely that line would even show up on the IMDB page had it not been in Taxi Driver.
From Home Improvement, Al’s catchphrase,“I don’t think so, Tim” was a throwaway in the pilot. And on Seinfeld, Teri Hatcher’s line,“They’re real, and they’re spectacular”, the second half was an ad lib.
Couldn’t have said it better myself.