Abandoned Hospital: Kaname takes Sousuke through one, which he treats as not much different than a bombed out building in a warzone.
Accidental Pervert: Happens to Mardukas in episode 8. He accidentally opens a door to a girl's locker room, then excuses himself immediately, but a rumor of a pervert running around the school causes Sōsuke to apprehend him. Hilarity Ensues after Sōsuke realizes who it is, and the mission Mardukas has for him, which adds more difficulty to a sleep-deprived Sōsuke.
Actor Allusion: Kaname muses that she'd like to install a device in Sōsuke's brain that would let her zap him whenever he did something embarassing. Kaname's voice actress, Satsuki Yukino, also voiced Kagome from InuYasha, who did much the same to the title character via his Restraining Bolt necklace. The allusion is lampshaded when Kyouko protests that Sōsuke isn't a dog.
Adult Fear: Played with in the "What Do They Fear?" Episode, in which Sousuke is shown to be completely unfazed by haunted hospitals, ghosts, horrific screams, and spooky children with hammers, thanks to having grown up in a war zone... but he is afraid when he thinks that his charge, Kaname, died falling through the floor.
Beware the Nice Ones: The janitor in episode 5. After Sosuke and Issei's competition result in him getting injured, they are ordered to take care of him. However, both go completely overboard in their attempts to win his favor, and cause even more problems. After they finally work together, they feed him a fish, and when they explain where they caught it, the janitor finally loses it, and goes Chainsaw Good on them. Despite Issei's attempts to fight him, and Sosuke both shooting and lobbing grenades, neither one is able to stop him.
Sosuke: (The next morning): Live ammo has no effect on this guy...
Cat Smile: Tessa displays one briefly when trying to sneak into Sōsuke's bed again in episode 9, right before Mao grabs her and handcuffs her.
Cluster F-Bomb: You believe that Sōsuke's outburst when defeating Gauron was bad? Then here's a little example (note: the stuff is censored so this is mainly speculative)...
Sōsuke: Once you survive my training, then and only then will you become a weapon! Until that time you lowly maggots are nothing but a bunch of cocksuckers! I despise and look down upon you. Let's make one thing clear: my job is to find the limp dicks among you and weed them out! I won't have a stinking pussy on our team keeping us from victory! There will be no laughing or crying. You are NOT human beings! You are KILLING MACHINES! If you couldn't kill, your lives will become worthless! You fuckers would be better off in a corner jacking off your meat! You want to lose just to stand out? Pretend it hurts to gain some sympathy?! You are pathetic loser scum! The best part of you ran down on the crack of your mother's ass and ended up as a cumstain on the mattress! Quit draggin' your feet, you fuckers! If you whimper, I will unscrew your head and shit down on your neck! That ball is your only girlfriend! You don't need a Mary Jane with boobs and a large backside! Think of it as a wet pussy and fuck her as hard as you can!
Of course he got it from "Mao's Pocketbook: How To Abuse A Training Recruit" (Mao, in turn, seems to have gotten it from Gunnery Sergeant Hartmann). Kaname speculates that he doesn't know the actual meaning of what he's saying.
Comically Missing the Point: In one episode, Kaname takes Sōsuke through an Abandoned Hospital. While her intention was to scare him, he's more worried about possible traps and ambushes. When one of the girls gets scared and runs away because she saw an old lady staring at them from the hospital, Sōsuke nonchalantly mentions seeing her too, freaking out Kaname. But he wasn't worried because the old lady was simply looking at them, and didn't have a gun or RPG, the first of many frustrations as she tries to scare him.
Crazy-Prepared: When Sōsuke is faced with a hand-to-hand combat with 3 karate fighters, he subdues the first one with a non-lethal projectile from a gun, the second one (after explained he can't use guns) with some sort of gas and for the third one (after being beaten by Kaname) he used a hand grenade as a decoy to bring him down.
Also, in the first episode, Sōsuke is asked to lay off all his weapons. Clang! Clink! Clang! and next shot from the camera shows him standing next to a heap of weapons almost half his size. (Including several grenades and a Bazooka).
Cry Cute: Tessa looks so adorable after she trips and drops a glass in the beginning of episode 9.
The chauffeur Washio is strikingly similar to Oddjob from Goldfinger, from his attire right down to his combat style, including his signature bowler hat throw. He also dodges a grenade with a slow motion Matrix-style backflip.
Sōsuke: (upon dropping all his weapons) I've complied to your demand. Akutsu: Where the hell did you keep all that?!
Groin Attack: Sōsuke repeatedly stomped on a man's groin while he was down and gave another 4 successive punches there.
He's also on the receiving end of one when Kazama accidentally headbutts him there in the Hot Springs Episode. He doesn't react at all.
Hero Stole My Bike: Kaname and Sōsuke steal a bike in order to get back to school in time for their test. Hilarity Ensues when a crazy policewoman tries to pull them over, leading to a Chase Scene that results in her crashing her squad car. Which then explodes.
Heroic Comedic Sociopath: Sōsuke. Without any personality adjustments, just removing all the mecha fights or serious drama, his behaviour goes from 'Fish out of Water' to 'Hilariously Over-The-Top Lunatic'.
Hey, Catch!: Sōsuke does one with a grenade with the pin still in to beat one of the karate team members.
Hostage Situation: Sōsuke imagines one in the 3rd episode. Ironically, Sousuke's vision in that episode is the only time in the anime when Kaname is kidnapped on screen (although she is kidnapped offscreen in other episodes).
Hot Springs Episode: Episode 9, complete with the antics of Kurz Weber, where he desperately attempts to peep within the girl's bathhouses. Although his plans are foiled each time by Sōsuke.
Intimidation Demonstration: In the episode "Summer Illusion of Steel", Sameshima the cook runs up to Sōsuke with a knife in each hand, and starts twirling them around. He then switches over to rapidly slicing them through the air while stating his reputation as "Sammy the Slasher". It pays homage to Raiders of the Lost Ark, though, as Sōsuke just shoots him.
Must Not Die a Virgin: Mizuki tries this on Issei, and Chidori also mistakes Sousuke's behavior for this. She should know better at this point...
Ms. Fanservice: In the seventh episode, next-episode teaser, Chidori accuses the show of using Tessa for fanservice.
Not What It Looks Like: Played straight until it's subverted, when Chidori has full knowledge on how incapable Sōsuke is on acting on anything even remotely connected to sex.
One Dialogue, Two Conversations: After Sosuke nearly drowns in the school pool, Tessa says she's going to perform CPR on him, saying that they have a special relationship due to being Childhood Friends, as her cover story to the other students. After Sosuke recovers on his own, the other students ask him if he has a "special relationship" with Tessa. Since he can't expose his or Tessa's real identities, and he thinks they're referring to his military relationship as her subordinate, he verifies it. Much Squeeing ensues from them, while Chidori storms off, telling a confused Sosuke to talk to Tessa, since they have a "special relationship".
Opposing Sports Team: The Garasuyama Rugby Club; they're classically big, intimidating, aggressive, and arrogant.
Pixellation: Used in a gag to hide the contents of a gory package.
Another to hide what looks like a rape scene (actually tickling with feathers).
The Rival: Sosuke to Tsubaki, Tessa to Kaname, and Bonta-kun to the policewoman.
Sequential Symptom Syndrome: Sousuke describes the effects of the trap he set as it shows the coach who set them off with the symptoms.
Shame If Something Happened: After Kaname is kidnapped by a street gang, Sousuke kidnaps the leader's little brother in turn and dangles him over them, threatening to drop him. To keep the other gang members from doing anything, he addresses each of them in turn by name, saying it would be a shame if their sister/girlfriend/sickly mother/lovebird/bike was to meet an...unfortunate fate. This thoroughly freaks out the gang members, who all run away in fear, leaving the leader, Kaname, and the brother who was in on the whole thing: Sousuke had bribed him to go along with it with a promise of an RC car
The reason for the odd position was most likely because Sousuke (subconsciously) associated Tessa's behaviour with torture (well, emotionally, it sort of was). The stripping and tying upside down is a common procedure of putting the tortured person in a position of vulnerability before any sort of pain is involved. Honestly, it's a classic scene in all books that depict war and/or espionage.
Shout-Out: In episode 11, the enraged model club member refers to the woman who insulted him as a Perfect Grade bitch.
Slasher Smile: The policewoman has one to make the Joker proud, whenever she gets caught up in the chase.
Take Off Your Clothes: In the last episode, after a seemingly deadly virus spread to the school, Sōsuke drags Kaname to the infirmary and asks her to take off her clothes. Kaname reacts all flustered, believing he wants to make love to her before they die. But Sōsuke learned that the "virus" only attacks synthetic fibers, including clothes, and he's just asking her to undress to save her school uniform before it'd disintegrate.