Okabe's reaction to meeting Ruka for the first time:
Okabe: Urushibara Ruka. Delicate as an orchid, fair as a cherry blossom, the personification of feminine grace. But he's a guy. Tall as a willow, slender as a reed... but he's a guy. Radiant in shrine vestments. But he's a guy. The sun dips low in a rosy sky, cicadas buzz... But he's a guy.
"Anata no banana puni puni da ne!" Your banana is so soft and squishy, by yours truly Mayushii!
In the True Ending, when Okabe encounters Beta World Suzuha:
Beta!Suzuha: There lies a worldline unaffected by any attractor field. A worldline knows as Steins Gate. Okabe: But that's the chunni name I pulled out of my ass! It doesn't even mean anything!
In Faris' ending, Okabe "loses" his ability to play Rai-Net (which he really never had, thanks to his Ripple Effect-Proof Memory)note In this timeline, Faris' father is still alive, Okabe and Faris met at a Rai-Net tournament, and Okabe never met Mayuri, Daru, or Kurisu, thus the lab was never formed. They try and play some games, which of course he does terrible at.
Okabe: I just said 'massacre everyone', so why is this woman running towards me? She's good natured, even though she's a tsundere.
In Chapter 8, after Luka's Love Confession, Okabe returns to the lab really in shock and says gibberish about the invention of the Microwave. Mayuri, who saw him has this to say...
Mayuri: Oh no! Okarin is broken.
Okabe has a habit of using very Visual Novel-esque descriptions for people he doesn't know. Such as stating out loud that he's going to call an un-named police officer "Officer A" (the dialogue box changes to match).
When Okabe finally tells Kurisu about the reasons for his repeated time-leaping, she suggests he uses a Trust Password to get her help quicker next loop. She picks something embarrassing enough that her past self's reaction is:
Kurisu: I'm going to kill myself five hours from now...
Moeka misspelling Hououin Kyouma as "Hoboing Coma" from Episode 2.
This talk from episode 3:
Daru: What do you want the text message to say? Okabe: How about "Christina is a pervert." Kurisu: Don't go pushing your luck, jerkwad. Daru: How about I just split the difference: "O-kar-in is a per-vert." Okabe: What!? Kurisu: Atta boy! Good thinking! Daru: Firing torpedoes, captain!
Okabe: Word to your mother! Suzuha: I cannot believe you just said "word to your mother" to this guy, you freak.
Kurisu recognizing that Okabe is a virgin.
Daru attempting to give Okabe dating advice.
Daru: Have you considered giving the wisdom between these pages a try? Okabe: That's a hentai game strategy guide, jackass!
Episode 23: Okabe describing his future self as "an otaku who read too many bad light novels". This is sadly lost in the dub.
In episode 23, Daru consistently being confused and looking for a different person in the background whenever Suzuha refers to him as her dad, thinking that she must be talking to someone else. Poor guy doesn't have a clue.
Episode 25: The return of Gratuitous English Okabe, only this time he tries on American airport security. It goes slightly less well.
What does Moeka want? "MACADAMIA NUTS MACADAMIA NUTS MACADAMIA NUTS MACADAMIA NUTS... anything but"
After Okabe questions Kurisu's driving, causing her to speed off from the airport without him, cut to Okabe trudging along the road, seemingly exhausted.
Kurisu: ...I watched you get out of the stupid cab, you big faker. [Okabe's taxi zooms past them in the background]
Kurisu in the cat maid outfit.
If you're a Los Angeles native, it's pretty amusing to see familiar places in this episode. The details of the Staples Center, LAX, and the LA Convention Center are spot on.
The Somei Eichi no Cognitive Computing shorts are usually very amusing
The first one has the computer upa AI thing help out Kurisu cook. Both Okabe and Daru react appropriately. It leads to them making chocolate burritos which Okabe tries to force Daru to try first. It ends in Okabe admitting they taste great.
The third one has Nae calling out Okabe and Daru on their limited wardrobe, causing them to ask Kurisu for a make over. When she has the computer Upa help. It creates some really ridiculous outfits for Daru. And for the audience and Kurisu's shock, some outfits that reallyfit Okabe to the point Kurisu can't breathe.
Okabe discovers, in one world line after he sends a D-mail - the hard way - that Ruka is now actually a girl instead of a guy wanting to be one.
Or Kurisu, earlier before the world shift, discovering that Ruka was a boy pretty much the same way. Cut to Kurisu leaning against a wall, looking ill, and Mayuri asking her, "You okay, there?"
EVERY time Okabe talks to his Imaginary Friend on his phone, to the point that he has to come up with Blatant Lies as to how his phone magically turns off when anyone besides him touches it when Kurisu grabs his phone and sees that it's off. While many of Okabe's paranoid and/or nonsensical rantings turn out to have some measureof truth to them, this is the one aspect of him that is purely fictional.
In the movie: Kurisu, in an attempt to jog the other lab members' memories of Okabe, pulling a comically-theatrical impersonation of his Hououin Kyouma persona.
Any time Daru keenly takes notice of a Double Entendre, since the viewers usually notice them at the same time.
Also, any time Daru compares what's happening in-universe to a LN or an anime, considering the show's source material.
Any time Moeka talks about how much FB means to her because to a lot of people interpret it as "Facebook"
Kurisu mistaking Daru's underwear for a lab coat.
Pretty much all of drunk Kurisu in the movie.
After receiving a text, Okabe runs back to the Lab, then walks in on Mayushii and Kurisu in the shower. He's booted out, and then pulls out his phone...
Okabe: It's me. We have a problem. I'm under attack from Mongolian Spot and Ditzy sisters. They're going directly after my sanity, a bold move against the insane mad scientist Hououin Kyouma. This is the strongest offensive I've encountered so far. I realize now that vulnerability itself can be a powerful weapon. They're pulling out all the stops, and I don't think my pants can take it. Give in to the temptation? Nonsense. I could never do that. They're fellow lab members, my loyal assistant and my childhood friend. ...Must I? Is it truly the will of Steins Gate that I give in? That I raise my hand against my closest companions? If so... then how cruel the fates be! El... Psy... Kongroo!
Okabe and Ruka's painfully awkward date.
The entirety of the "My Darling is an idiot!" segment counts as one. From Okabe's and Kurisu's mutual Tsundere tendencies making the whole situation one huge bicker fest, to the fact that the "World Ending Data" that the crew is trying so hard to keep from broadcasting turns out to be a sickeningly sweet dream of her and Okabe.
Kurisu calling Okabe an "airhead" in a D-mail (written in English). Okabe wonders what that means, and she tells him it's a compliment in America. He later looks it up and gets outraged.