Kagami: What's with the (bleep) D-like driving!? note The "bleep" was intentional. It's part of the dialogue.
Or how about in the same episode when, after Tsukasa puts a seashell to her ear and imagines she's floating in the ocean, a roach pops out, prompting a very reasonable scream and making her run to her sister?
Another favorite is in the "Wawawawasuremono..." ending, which includes Shiraishi singing next to a random couple (who seem to be trying their best to ignore him), and a subtitle pops up pointing at them which reads: "Random couple who readily agreed to be in this video."
The New Years episode was memorable because out of the blue, Tsukasa says some odd dialogue ("Rice cake goes wee!" and "Cheesecake?").
After Konata's famous "a flat chest is a status symbol" scene, Tsukasa replies "I guess I'm safe too".
In episode 21, where they go to the Daibutsu in Nara, and Konata calls the figures around the main statue "the options".
Konata: Hmm... if this dude's the main one... then the little ones around him... are all the Options!
The spectacular Mind Screw at the end is a mixture of this and Nightmare Fuel. The Deranged Animation and nonsensical dialogue is already bound to get a few laughs, but it especially starts turning into a mindfuck when the girls are presented two particular frogs: Keroro and Tamama. They keep chanting their names louder and louder, and next thing you know, the girls seem to have turned into different frogs themselves, with Kagami being partially embedded in the floor. As Konata is wondering what happened to everyone, the scene unzooms to reveal a small, chibi version of Minoru Shiraishi dressed as a wizard; he sings "Wawawa wasuremono..." as he leaves the screen, ending the sequence.
A blink-and-you'll-miss-it moment in the MMORPG part of the OVA: When Konata and co. do a team victory pose, a random male character joins in on the far right, and then just walks away without anyone noticing.
Right after Konata's trying to think of a good nickname for Kagami, the latter, trying to get her to stop, trolls her with 'just call me Kagami-sama', or 'The Great Kagami', as it's subbed as. Cut to later that day when Konata visits Kagami's classroom, full of students, so that they can walk home.
Konata: (waving) Oi, Kagami-sama!
Kagami: (blue-faced) Oh my god, she actually said it!
Konata explains Kagami how she met Tsukasa: A foreigner (Who bears an intentionally uncanny resemblance to Guile) was about to kidnap Tsukasa when Konata showed up and beat the crap out of him Street Fighter-style. Tsukasa corrected her by precising that the foreigner was actually asking her for directions.
The moment when Kagami, in the bread eating contest at their school's sports day, overcompensates by biting the bread and bringing the WHOLE RACK it was on crashing down.
The concert had two, both from Minoru — dancing with a Nyamo plushie during "Ore no Wasuremono" and crossdressing as Miyuki in the last several minutes to make up for Aya Endo's absence (Tomokazu Seki even tries to flip his skirt during the curtain call!).
Watching the English subbed version as a non-Japanese speaker can pull a few laughs, with Japanese words sounding like English words (Cookiecookiecookiecookie)note The onomatopoeia Konata makes while she's attempting to draw something, as well as the genuine Gratuitous English.
Konata has a discussion with her dad regarding Lolicons. When Sojiro insists that he is also into normal girls, which means he's not just a lolicon, but a guy who is also a lolicon, Konata states that he fails as a human being either way. Ouch.