Donna getting Louis to panic and run away from trying to get his reward from Harvey for doing Harvey a favor.
Donna: (fake crying)
Louis: Oh shit... (runs away)
Also, Harvey's realization that Donna used the same trick on him before to get him to let her parents stay in his condo
A Call Back moment occurs when Donna begins crying in front of Mike as evidence of her excellent acting skills. Mike, of course, panicked too.
Mike borrows Harvey's corporate credit card for use in a case:
Harvey: If this comes back to me smelling like stripper, you're fired.
Harvey telling Mike he's getting really good at ... ruining the moment.
Mike commenting that he was a fabulous criminal, only for Harvey to remind him about how his briefcase fell open and then there was weed all over the floor.
Any time Jessica brings the snark:
The really important thing is - that I'm taller than you.
And when you're running the firm of Litt, Nothing, and Nobody...
Harvey: You could teach-
Jessica: I could also kill myself.
Every time Mike gives Harvey an idea, only for him to reject it. A few scenes later, Harvey inevitably does the exact same thing Mike told him to do, insisting that he did not say it was a bad idea.
For some unknown reason, Harvey has Michael Jordan's phone number. Mike calls his bluff, dials the number, and promptly freaks out as the call actually connects.
Fridge Brilliance when you remember that Harvey keeps various signed sports memorabilia in his office among which are basketballs.
Turns out MJ is a client of Harvey's.
Louis's Hidden Depths? Going to the shooting range when he wants to be alone. Harvey decides to quote Dirty Harry to him:
A .44 Magnum? You feelin' lucky today, punk?
Apparently Harvey's pre-trial ritual involves Donna, at least five minutes, and a can opener.
and, somehow, thumbtacks
Louis's reaction to the tickets to the American Ballet Theatre that Donna bribes him with. His eyes look like friggin' saucers.
And then when he turns them down and Donna reveals Jessica already knows about his sandbagging the DNA test:
Louis: Oh, that phony bitch.
Mike's dinner with Jessica at the beginning of Season 2:
Mike: You look so young!
Jessica: Well you know what they say...
Mike: Black don't crack!
Jessica: (simultaneously) True beauty is ageless.
Mike: ... That's what I meant to say.
Mike's grandmother reveals that one of her stories she told Mike was actually fake. As he's leaving, he jokingly asks her if there's any other secrets she hasn't told him such as if he's really from Krypton. She simply nods slowly and says "Possible..." with the straightest possible face.
Harvey and Jessica make the same joke about redundant departments to Louis. Louis naturally has completely different reactions to the two of them.
This exchange between Harvey and Jessica:
Harvey: There can only be one.
Jessica: We're at a funeral and you're quoting Highlander?
Harvey: Yeah, a lot of people die in it. Seems appropriate.
When the firm ends up being threatened with having their rights to the pick of Harvard graduates revoked due to their Training from Hell attitude towards interns, Louis hires an inspector to rectify the problem...who turns out to be nearly an Opposite-Sex Clone of Louis himself. Hilarity Ensues.
In the same episode, Louis assists in one of Harvey's Eureka Moments. Harvey will only say it once, but "Louis, you're the man". Louis's subsequent mouthing of "what. the. fuck?" after Harvey leaves is hilarious.
Anything involving Bruno. Even his euthanasia at the end of the episode is Black Comedy due to Louis's overblown reaction.
Harvey meeting Mike's grandmother and they start talking. His reaction to the below quote is priceless.
Harvey: Your grandson says nice things about you.
Edith: Oh, he tells me you're quite the hardass.
Harvey and Mike getting stoned together. Whatever isn't Heartwarming is hilarious, topped off by the two planning to sneak into Hardman's office to urinate on his carpet.
When Donna is giving a clearly bummed out Harvey a friendly You Suck speech over Mike not coming into work, she notes that unlike Mike, she doesn't have room for an eidetic since her head's already full of awesome in other ways.
Donna and Harvey's prank on Louis. They plaster his room office with his mugshot and captioned with different statements like "Major asshole". This includes replacing his photos, his files, everything with mugshots. And to top it off, they superglue his chair so that when he stands up, his pants rip. The best part? Not his Oh, Crap reactions. It's that he's actually happy about it because it means Harvey isn't (mostly) pissed at him.
Louis seeing Nigel kiss Donna's hand at the elevator.
Mike, lowly associate, runs into Darby, head of a multi-national law firm outside Harvey's office. They get along quite well, especially on the topic of Downton Abbey.
Louis is having a great morning and he steps out of the elevator and sees Harvey's name added to the firm's. His expression is priceless and he ends up taking a day off to recover from the shock. Later in the episode we see a flashback that shows that he had the exact same reaction to meeting Donna for the first time.
Louis: I eat cocks for breakfast, lunch and dinner....and it's delicious.
When Mike returns to the firm in Season 4, Katrina and him get into a little bit of a insult war (as befitting two proteges that take after their mentors). They're actually pretty happy to see each other but their history makes it very awkward to express any sort of friendship. It culminates in an an even more awkward hover-hand airhug.