Yugi's response to Grandpa reading the hieroglyphs on the puzzle box. Grandpa reads 'whoever solves this puzzle will receive my dark knowledge and power' and Yugi's response is that his wish for friends will come true.
Kaiba offering Yugi's grandpa every card in his briefcase in exchange for a Blue-Eyes is met with an overdramatic sting... as is Grandpa's simple response of "No."
In the third episode Kaiba, not wanting to lose to Yugi, forces a draw. Then he rages about getting a draw and vows to beat Yugi.
In episode four, Honda asks Yugi to fill in for him in a line because he needs to pee.
The sheer amount of Serious Business in the yo-yo gang episode is hilarious, especially since unlike the manga they aren't weaponized in any way except for Hirutani's, so his gang is terrorizing Domino with ordinary children's toys.
Kaiba dons a hilariously dorky disguise when kidnapping Yugi in episode 8.
In episode 17, Anzu slaps Yami Yugi so hard he turns back into Yugi.
During the Monster World game, Yugi and friends find a painfully obvious trap inside a tower. When Dark Yugi realizes it's a trap he tells them to get out... and finds they're already inside the tower.
Zorc's Dark Catastrophe is represented as a huge, devastating attack in-game that blasts everyone away. Out-of-game, it's seen as the figurines gently falling over.