An early incident
First, let's start with bit of history. Sometime in the year 2000, an internet humorist named Richard "Lowtax" Kyanka, proprietor of the SomethingAwful humor site and associated forums, created a character, Jeff K. This character was a teenaged would-be hacker but generally inept with computers. He spoke in a deliberately agrammatical and misspelled idiosyncratic dialect, using phrases such as: SINCE I AM TEH LEADERING JONRALIST OF TEH COMPUTAR GAMEING SCENE, I HAEV WRITTAN A REPORT ON WINDOWS WHISTLAR.
This shtick was pretty funny (at least for a while, and if you happen to like this variety of humor), but what was much less funny was that hundreds of people on the SomethingAwful forums now started talking like JeffK, in the mistaken belief that, because Jeff K was funny, if they would only spell "the" as "teh" and "you" as "yuo" and maybe use some incorrect grammar, they too would be operating at the pinnacle of internet humor. It turns out that this is not the case. In fact, it was so annoying that Kyanka began to ban people parroting these catchphrases, or at least to discourage them by adding word filters to the forums that would replace the annoying catchphrase of the moment with something else, such as "I AM AN UNFUNNY FAGGOT", every time someone tried to post it. This came as an unwelcome surprise to many posters, especially newer ones, but a bit of reflection ought to make it fairly obvious in hindsight.
Quoting jokes is often annoying
It's difficult to pull off stolen jokes. Comedian Dave Chappelle is pretty funny. He comes up with a lot of amusing skits and lines. You probably know someone, however, who thinks Chappelle is so funny that they insist on saying "FUCK YO COUCH NIGGA" every time an opportunity arises. This is not that funny, especially after one or two times. Now imagine that, every few months, a new group of people start saying "FUCK YO COUCH NIGGA", thinking they've just discovered the cleverest thing in the world. This is basically what happens with recycled internet catchphrases. If you're a computer scientist, incidentally, you're probably already familiar with the non-internet version of this phenomenon in the form of Monty Python quotes. Monty Python may be funny, but after years of it eventually you're going to snap and kill the next motherfucker who says "ni".
Did that guy really just say "pwn" out loud?
But, back to the internet. Fast-forward a few years, and now the unfunny morons in the SomethingAwful forums have spread to large sections of the internet. Some of them even venture into the real world, compounding the problem. Initially, the offline problems began with gamer nerds. You might recall the first time you heard someone say "pwn" in real life (pronounced "pone", apparently), and staring at them with jaw dropped wondering if they really seriously fucking said "pwn" out loud, and hoping to catch some hint that they were being sarcastic. A few years later and pwn is the least of anyone's problems.
A brief guide to self-improvement
So how do you avoid this sort of failure? (The failure is merely annoying, incidentally, not particularly epic.) A rule of thumb is that if you're going to parrot other people's jokes, you should at least not parrot the same jokes everyone else is also parroting, especially if they have been doing it for a long time already. When it comes to internet catchphrases, this includes all of them. In real life you might take some trend from one group of people, introduce it to a new group of people, and be seen as clever. However you are not unique in having access to the internet, so unless you invented something or aren't that many degrees of separation from the place where it was invented, you should assume that tens of thousands of people have already beaten you to it. You don't want to be the 150,000th person to jump on some lame trend.
In practical terms, if anything you're contemplating saying appears on Encyclopedia Dramatica, in the Urban Dictionary, on Fark, on random blogs, or in the vocabulary of your nerd friends, the joke is probably at least two years old and you should avoid it. Often they're 5+ years old, being basically watered-down versions of SomethingAwful jokes that may once have been mildly funny but are now so stupid as to be ridiculous. Even if you happen to hit on a relatively fresh trend, these are sort of the dregs of internet humor, purveying the most insipidly boring versions of "edgy", "random" humor, analogous to getting your rebellious, noncomformist fashion from Hot Topic. And of course, the only thing worse than humor that gets repeated so much that it's no longer funny is humor that's repeated a lot despite never having been funny at all. This is worse than saying "pwn" out loud, so don't do it.
Saying things that have not been said before
All is not lost, however. If you think a little, it is actually possible to make humorous comments that aren't lifted directly from 100,000 other people. If you try really hard, you can even go for jokes that are actually funny. Humorous allusions and injokes are fine, too, though fresh ones are better than the tired variety. Just have a little creativity and originality, please.