The Nigerian Prince scam goes back at least as far as the 16th century. It was called the Spanish Prisoner scam back then and send out through regular mail, but everything else is exactly the same.
In-universe (or possibly lampshaded) example. The Nostalgia Critic did a review of Transformers: Revenge of the Fallen, he acted out all of it like he did in his review of the original TransformersLive-Action Adaptation. That first review was the basis for the later character of Chester A. Bum. At the end of the second review, Chester A. Bum leaned into frame and said, "Dude, did he just steal my act?"
The Critic didn't actually coin the phrase "You know, for kids!", which he says whenever something suggestive happens in a kids' movie. It's a quote from The Hudsucker Proxy.
Possibly the example with the shortest amount of time, Lanipator had to put up a disclaimer that he did not steal the "Neighborhood Watch" joke from Naruto The Abridged Series. In fact, he used it first, and Naruto Abridged used it as a homage to him. Which should be obvious, considering the neighborhood watch van is clearly from Yu Yu Hakusho (given its art and color style)
Thuddingly literal example: almost any comedy video on YouTube that references current pop culture will have comments added years later about how the reference is so dated or played out, in the delusion that all videos they haven't seen yet must be new. Direct parodies seem to suffer from this most heavily.
Many people that hear the Microsoft Sam voice being used for anything instantly associate it with Master Chief of the Arby n' the Chief series.
Long before the internet-famous LongCat, there was a similarly long Siamese cat in an episode of Monty Python's Flying Circus eating Killer Cars and normal buildings (would that make it a Tacgnol Esemais?).
While the trollface meme as we know it did indeed begin with the famous comic, the facial expression itself of the squinted eyes and the smile turned up at a corner is a trademark of Ernest P. Worrell. It took off in the context of trolls from the movie Ernest Scared Stupid, in which Ernest makes the face while teasing an actual troll, but it isn't always used when he's trying to annoy someone, and it predates the comic.
Shit Pickle and Super Ultra Mecha Death Christ 2000 first appeared in an animated short film by James Rolfe called The Wizard of Oz 3: Dorothy Goes to Hell. However, they're usually thought of as characters from The Angry Video Game Nerd, where they made cameo appearances.
If you say the word "trope" outside of TV Tropes, you may get attacked by TV Tropes' Hate Dumb or just assumed to be a troper.
On the flip side, good trope names are often pre-existing terms, some of them ancient.
Roger Ebert made lists of tropes similar to TV Tropes, usually for snark purposes.
The "at" symbol (@) has existed since at least the 1300s. 
Sadly this 4chan board or omegle did not create the line, "hi, welcome to stealth," that right goes to Bad Religion with their song "Stealth" off of their album Recipe for Hate made in 1993.
SCP-173 is a statue that moves when you aren't looking at it and tries to kill you. It's easy to assume that the Foundation found a Weeping Angel...but, as the FAQ points out, the entry was submitted a few months before that episode aired.
The term "brony" did not originate in My Little Pony: Friendship Is Magic, but is a lost place-name, now preserved in Bronington, Shropshire, meaning "people of the islands amongst the stream", from either Old English burna 'stream' (or more probably) Old Norse brunnr "spring, stream" along with Old English -ing "people of" and tun "farm", or alternatively Celtic *brynn "hill", Old English ieg "island" and tun "farmstead", meaning "hill-isle farm". A record as Brunynton and as Bronintone. So, any fans thinking they invented the word... it's not as new as some people would think.
Matt and Pat of Two Best Friends Play turned the phrase "Do you know how many [object] I have to [verb]? More than you'd think, and less than you'd hope," (and its many, many variations) into a Running Gag. They first used it in their Soul Caliber V video, which they uploaded on February 26, 2012. In a July 2013 video of a Deadly Premonition playthrough, they noticed that one of the characters in the game says, "I know more than you'd think, but less than you'd hope." Deadly Premonition was released in 2010. Matt and Pat were flabbergasted to realize they'd been unwittingly copying DP that whole time.
Doomworld is often claimed to be the oldest unofficial Doom news site. In fact, DoomGate arrived in 1994, not long after the game itself, and is still running today, albeit no longer updated; Doomworld wasn't launched until 1998.