From the first chapter, Keroro recalling rule eight of the Keronian Military Code ("The penalty for allowing advanced technology to be taken by the enemy... is death!") after he notices his Kero Ball is missing... and immediately trying to think up a loophole.
Keroro: No, not taken. Definitely not taken! I merely lent it to them!
We meet Tamama for the first time!
Keroro: Private Tamama is...
Tamama: One fly in my sights, sir!!(Shoots mouth beam)TASTE MY PAIN, INSECTOID SCUM!!!
From the pool episode, Natsumi getting Skinship Groped by her (female) classmates, and the author apologizing for getting carried away.
At the climax of chapter 129, Natsumi starts melting the Keronians' iceberg base with a huge laser cannon:
Tamama: At times like this, if you watch the characters' sweat, you can see how tight the spot they're in is, yes! (glances at Keroro, who is sweating bullets and wearing a blank expression)We're up the creek!
In one chapter, there's an athletics festival at Fuyuki and Natsumi's school. Fuyuki is unhappy because he A) sucks at sports and B) is under a lot of pressure as Natsumi's brother.
Fuyuki: It's not fair, why couldn't they have an occult festival...?
Caption: Because that would be silly. Even aliens know that.
There's one scene in Episode 20, where the frogs and Fuyuki, a noted hydrophobic, are exploring the ocean near the beach and encounter a mysterious underwater race. Keroro ends up launching the missiles at the race (instead of reeling Fuyuki back in) leading to the onscreen text to read "Atmosphere of 'Oh, shit'".
The Funimation Gag Dub pretty much has one of these lines per episode. Such as...
Keroro: Not that I'm not also happy, but how did you pass the bombs?
Natsumi: Your bombs...* holds out a leek* I just used the Leek Spin!
Keroro going bonkers with a juiced up vacuum cleaner, de-skirting Natsumi, and proudly hoisting the said skirt as a victory flag on his vacuum.
Equally funny in the manga, where he used the vacuum cleaner to steal Natsumi's panties.
Giroro literally blasting through the special ending credits of episode 150.
Angol Mois calling the radars "boobies" in the dub of episode 27.
Mom's explanation for why she came home early in Episode 1: "Well, the writer said that he thinks better in his underwear, and I wasn't gonna stick around for that."
In episode 147-B, Giroro receives chocolates from an unknown admirer. He uses the process of elimination to discount all the main females until Natsumi (who he'd hoped gave him the chocolates, with his crush on her and all) is the only one left. He decides to thank her personally...at which point Natsumi reveals herself not to be the one to give him the "chocolates", which are revealed to be curry, and that the real sender was Kululu.
Fuyuki's conversation with Mr. Nishizawa in the dub of episode 62a:
Mr. Nishizawa: They tell me you're quite the detective at the school you go to. A regular Conan O'Brien. Fuyuki:You may have your Conans confused. Mr. Nishizawa: I'm sure a barbarian like you and his frog friends can help me. ... Mr. Nishizawa: This painting, Venus's Arrival, is incredibly important to my daughter. You must use the frogs' advanced skills to protect it. It is imperative that I keep my daughter happy. Fuyuki: OK, but everyone knows how demanding you are. If I fail, are you going toshoot me? Mr. Nishizawa:Yes, probably.