Given that Arino's a comedian, things getting funny is only a given.
In the first attempt at playing the ludicrously hard Super Mario Brothers: The Lost Levels, Arino makes it past level 8-2's "Triangle Paratrooper Jump" (Where you need to time a jump on a Paratrooper just right such that you hit a block to open a way to the last part of the level (in the form of a vine), needless to say, this took up a fairly large chunk of the challenge) only to miss the last jump, just centimetres away from the end level flag. The pure inhuman scream that Arino emits is hilarious.
Before starting on Lost Levels, however, he's shown attempting the original game, and failing to do the infinite 1-ups trick. After repeated failures of it, the episode cuts to two-and-a-half hours later where...he's still trying to get infinite 1-ups.
In Lemmings, after figuring out that you can't save all of the Lemmings if you use some as blockers, he starts naming the unfortunate lemmings after ADs. This got silly right about the time he was screaming "TOJIMA!"
Then, at the end, he started extending the challenge past what was called his "Last Lemmings." The name evolved into stuff like "True Last Lemmings" before finally winning on "Return of Last Lemmings."
While playing Balloon Fight as Satoru Iwata watches, the Nintendo president explains to him how things in the game work, such as the fish who is always moving around the screen and so may or may not appear when the player goes too low. He then says he knows those things because he programed the game.
Iwata later tries to catch 20 balloons in a row. He does well but fails, and thus Arino is the one doing snarky praises to the game's maker: "Well done, sir! You need luck to stay up. Nice! That was very clever."
During the Dig Dug II episode, Arino goes to a toy store in which he plays slot car racing with the staff. Near the end, his car flips over and... is pushed to victory by the second car. The third car then also jumps over to push them.
In the Punch-Out!! episode, AD Emoto offers to fight Soda Popinski, with Arino serving as his "second". Every time Mac gets knocked down, Arino throws in the towel. Emoto ultimately loses by TKO, and just to further rub it in, Arino laughs like Popinski.
Ghouls 'N Ghosts: Arino makes it to the stage 1 boss much sooner than previously (contrast to Ghosts 'N Goblins, where it took him 5 hours to do the same), and cheers that he's about to clear the stage... only to get a glimpse of the huge boss and immediately run away screaming.
The live Takeshi's Challenge challenge featured Inoue repeatedly doing an impression of Takeshi himself, much to everyone's chagrin. Then, during the karaoke part, Nakayama tells Inoue and Arino that all they have to do is sing really loudly. You can guess the result.
During the journey for the northernmost arcade game in Japan, we're told they didn't find any games in one town, so they'll keep moving north. Arino has this silly yet sympathetic look of great despair, which is how anyone would feel when visiting malls and stores in search of arcades and game kiosks, only to find none.
The Game Center CX in Asia episode features Super Mario Kart for the challenge. He isn't able to clear the objective of finishing every course, so he plays the usual 5-round match against the staff. The last match is against now-director Inoue, who is on par with his skill (read: low), on Rainbow Road. Even the narrator foreshadows the chaos that predictably ensues.
On the DVD cut of the Ninja Gaiden episode, as time runs short on the first day of the challenge, Toujima warns Arino that "this is no laughing matter," to remind him of his schedule. Arino answers by making a fart sound, instantly destroying the serious mood Toujima was trying to set.
The Super Mario Land challenge. Kibe forgets the script for the episode, and the first part of the episode is a massive running gag about it. Later, Arino decides to call Kibe, who reveals that he completely forgot it because Takahashi was distracted during their meeting. In addition, Kibe spoils the length of the game to Arino, followed by a blunt "Wait. Was I not supposed to say that?" Everyone's reaction is priceless.