Postman: We have an order for Ms. Shirai Kuroko. Mikoto: What's the contents? Postman: Computer parts. Kuroko:[aside to audience] Phew, thank goodness these companies are always discreet about their products. Mikoto: Who sent it? Postman:Love Potions and Aphrodisiacs Incorporated. Kuroko: Why the hell did they bother to label it "computer parts" then!!
Kuroko's reaction to hearing that the curse of the Undressing Woman is infectious.
Pretty much any scene with Kuroko that, in some way, involves Mikoto.
Episode 3 has several very funny bits: The reveal of what the villain has been doing to the girls she is attacking ("I have photographs, but it's not pretty. If you're going to look, prepare yourself."), the villain attempting to use the stun-gun on Mikoto, and the villain's Motive Rant.
Saten: Uh...what? Mikoto: Yeah, you lost me somewhere in the middle there.
Anything in Episode 4 that involves Mikoto and Touma. His reaction to being told the score will be settled when Mikoto wins, her coming at him with a chainsaw made of vibrating iron sand, her reaction to trying to shock him directly (through his right hand), his hilariously bad attempt to give up, and the fact that she then proceeded to chase him around for the entire night.
Uiharu feeling so happy and in ecstasy of experiencing her dream of what it's like to be a Tokiwadai student during the mid-summer festival... until Saten decides to wake her up back to reality by flipping her skirt!
Practically any scene where Saten flips her skirt is hilarious. One particular scene is in Episode 9, where Uihara is standing around, noting that Saten is late. Only for Saten to flip her skirt from behind (in the least subtle manner imaginable) and announces loudly "Today's blue stripes huh?" It's all topped by Uihara crying and waving her fists in the air.
Kuroko getting mad at Kiyama for undressing in public at the cafe. What makes it funny is that Kuroko is talking about herself on how "Women with skewed sexual interests" gets interested in seeing Kiyama's naked body. In the manga Saten also joins her saying that no women should show her panties. While Misaka and Uiharu looked on.
Kiyama undressing in public in general are crowning moments. The very first time it happens, people mistake Touma trying to get her to put her shirt back on for him taking it off; he runs away screaming "It's not what you think!"
During the cafe scene, Uiharu and Saten are told she's a brain specialist.
Saten: Is there something wrong with Shirai-san's brain?
Episode 13. The gang is modelling swimsuits in a holographic studio. Once they're done, Mikoto stays behind to try on the kiddy swimsuit she avoided before, and accidentally broadcasts everything on the giant billboard outside. Touma sees it and goes, "You Have Got to Be Kidding Me!"
Episode 3 of Railgun S. Mikoto getting smacked in the head with a binder and Kuroko glomping Mikoto's giant teddy bear because it smells like her. Also Saten's reaction to Eri finding a stack full of cash cards.
Mikoto's interactions with 9982; the argument getting interrupted by the ice-cream truck man, and 9982 stealing Mikoto's ice cream, then narrating her Blatant Lies about not eating it and the narration of her talking Mikoto into getting them tea, then Mikoto putting the Gekoto badge on 9982 and the slap fight when she wanted it back.
Shokuhou: Don't start running with all your might... Mikoto: I thought I was only jogging.
Also in this chapter, Misaka and Shokuhou getting into an argument and Misaka pointing out Shokuhou's eyes.
Mikoto: And what's with your sparkling eyes? Do you think this is a shojo manga? Shokuhou: I was born this way! Weren't you ever taught not to make fun of other people's bodies?
At the beginning of the Daihaseisai, the managers are tasked with getting two of the Level 5's to introduce the games with a speech. Shokuhou agrees readily enough, but asking Accelerator, Kakine, and Mugino goes about as well as you'd expect, Misaka's unavailable, and nobody can find any information on #6, so in desperation they are forced to ask GunhaSogiita. He forgets the speech in the middle of it, so he decides to improvise and ends up doing a ridiculous speech in which he inserts the word Guts! everywhere he can.
In a Railgun omake chapter, the male researchers from the Level 6 Shift project need the clones as close to the original as possible, so they try to figure out what kind of panties Mikoto wears under her shorts. A request for Tree Diagram to calculate a simulation is immediately denied ("Don't use me for stupid things"), so they send in the Hound Dogs, a black-ops mercenary squad. They break into the Tokiwadai dorm in the middle of the day in full gear...and are immediately slaughtered by the dorm supervisor. Just as the researchers are declaring that they'll have to take to the field themselves, the female researchers reveal that they just went out and bought 20,000 pairs of cheap panties.
In one of the non-canon Railgun omake chapters, Mikoto is talking to a newly amnesiac Touma about their fights. However, the way she says it sounds like something else altogether ("Every night, you chase me around and do This and That!"). Touma comes to the horrified conclusion that he's been raping a middle-school girl on a regular basis...so he immediately asks her to marry him.
Episode 17 of Railgun S.
Kuroko gets freaked out when Mitsuko buys mice to feed to her pet boa.
Kuroko and Frenda get into a childish argument about whether Mikoto or Mugino is the best, leading to the former two groping the latter two's breasts and getting beaten up.
Kuroko bought turtle blood due to its fabled aphrodisiac properties. Naturally, Mikoto fries her.
Saten enters full-on Troll mode with Misaka, latching on every opportunity to tease Misaka about the cookies she made for Touma.