There's always someone who's constantly proving he's "that guy." From awkward phrases to rude insults when he's trying to be funny, there are many "that guy" behaviors.
Mr. Right

“A friend of mine when asked to pick up something very specific will never get the right thing. His excuse is ‘but this is better’. Also does this when asked to perform a task, often does something else.” Source
The Gunner

“So I’m a medical student, and just saying that is ‘That Guy’ behavior, but I’m bringing it up for a reason: in med school, we have a category of people called Gunners. Gunners are called Gunners because they rise to the top by gunning everyone else down around them. They provide misinformation about assignments, backstab you to your classmates, advisors, residents, attendings, nurses (whoever will stop and credulously listen), they jump in front of you to answer questions they know but are suddenly anonymous when you need help, they do only the things that make them look good and don’t take care of the people on their team, they never own up to their mistakes and try to hide failures at any cost, etc. etc. etc. A lot of these examples are not specific to medical school or even the field of medicine, and they are all representative of That Guy behavior. Don’t be a Gunner; don’t be That Guy.” Source
Mr. No Headphones

“‘That guy’ that listens to music without headphones in public. F* you.” Source
Mr. Easily Offended

“Someone who makes fun of you in front of your friends ‘as a joke’ and is constantly rude to you, yet the second you do it back to them, they flip s**.” Source
AKA High School

“‘That guy’ that is supposed to be your friend and is really cool when you hang out one on one but when you get around mutual friends, he acts like a dik to you. That f* guy.” Source
That Guy

“Everyone is accidentally rude sometimes. Most people will have a moment or two when they’re overly aggressive or pushy. Most people will make a joke at someone’s expense that isn’t appropriate. Nobody is perfect. A normal person, when called on their behavior, will apologize. A normal person will make an effort to change for the better. Not THAT GUY, though. THAT f**king GUY will accuse you of overreacting. He’ll say, ‘it was just a joke, man,’ as if he had done nothing wrong. In his mind, he can do no wrong. If he hurts someone, it’s their fault for feeling hurt. If he is rude, it’s justified because everybody just needs to ‘chill out’. Mock acoustic guitar Wonderwall guy all you want, but if he’s nice to my friends and listens when someone has a problem with him, he’s invited.” Source
Mr. Lie

“Lying under very unnecessary circumstances. -‘Didn’t you say you’d be here at 5?’ -‘Dude you’ll never believe it, my cars axel broke in the middle of the stoplight. It was a disaster’ He was driving his car the next day.” Source
Mr. Conceded

“My friend brought her new boyfriend to a party to introduce him around. He was a ‘DJ from Chicago and was soooo cool’…..this dude made a silhouette print of his own face, put it on a t-shirt and wore it to this party in total seriousness. He also wore sunglasses inside at night and it definitely wasn’t to be ironic. I kinda hate the phrase ‘that guy’ because I think its overused but I felt damn comfortable bringing it back out that night. Nobody could get over this. He wasn’t on drugs. He wears the sunglasses at ALL times. I hung out with him many other times outside this party because he was dating my close friend. Guys, this guy put his own face on a t shirt. If it were a blatant image of his face I’d say ‘ok haha he’s trolling’ but its his silhouette. It takes you a minute to realize it. That’s not trolling. That’s dead serious. Oh and he turned off the music at the party to play his OWN music. That he wrote. If this isn’t ‘that guy’, I don’t know what is.” Source
Mr. Perpetually Late

“Saying you’ll be somewhere in 5 minutes and are on the way. Then don’t show up for another hour. Perpetually late people: ‘I’m on the way’ = ‘I think I might be leaving soon’ ‘I’ll be there in 30 minutes’ = ‘I have just begun to get ready and once it only took me 5 minutes so that will surely happen this time.’ “Source
Mr. Bud Light

“Not bringing beer to a party, but drinking everyone else’s or the cousin of that move, bringing sh*tty beer to a party, throwing it in the cooler and drinking the better beer everyone else brought.” Source
A Giant Pot Of Potatoes And Lies

“The a**hole at the buffet who takes all the steak bits out of the stew, leaving behind a giant pot of carrots and potatoes and lies.” Source
The Mooch

“‘The guy’ that bums stuff from people. I actually know a guy coincidentally named guy who is the biggest mooch.” Source
The Borrower

“‘That guy’ who always has to ‘borrow’ something. Whether its a pencil, piece of paper, lighter, etc, you are not getting it back.” Source
Mr. Unoriginal

“Those people that yell out ‘RUN FORREST RUN’ at people running on the street.” Source
Schrodinger’s Douchebag

“Schrodinger’s douchebag: A guy who says really offensive things and decides whether or not he was joking based upon the reaction of people around him.” Source
I Don’t Want Your Advocare

“People that got suckered into a multi-level marketing scam and just won’t admit they got taken, so they just keep on trying to sell you on the secret to making money from home and great health while they slowly descend into abject poverty. Yeah, all of your FB friends hate you.” Source
Mr. Awkard

“Had ‘that guy’ around while camping once. It’s about 1am, fire is dying down, everybody is tired, conversation dies off and suddenly ‘awkward’.
Like, no, not awkward, I was admiring the f*king star and thinking about hitting the sack. and you ruined the peaceful moment. F you, guy.” Source