The fact that the narrator feels the need to re-introduce Racer X as Rex Racer every time he appears on screen, but sometimes the narrator re-introduces him as Rex Racer several times in the same episode.
The Motor Mouth dubbing crosses into this territory really easily.
Pops (to Rex): You're much too young yet! Iforbidyoutobeaprofessionalraceruntilyoureolderandhavemoreexperiencebehindthewheel!
"The Great Plan":
This line in the English dub during the motorcycle chase:
Speed: Hey, stop! Stop or I'll stop you!
"The Secret Engine":
Speed's uncomfortable reactions when "Lana" is flirting with him.
"The Race Against The Mammoth Car":
The exhaust of the Mach 5's Frogger mode having the same sound as a toilet flush.
"The Desperate Desert Race":
Rival of the week Kim orders the movers to load his car on the plane before the Mach 5. The movers' response? "Oip."note They most likely said "はい" (romaji: "hai") in the Japanese version (the Japanese equivalent of "Yes, sir/ma'am"), but Lord only knows how it turned into... this.
Spritle and Chim-Chim dressed as a mother and son bump into a large woman. She picks up Chim-Chim and realizes that he's a monkey. Her over-the-top reaction is hilarious.
Speed and a girl named Twinkle Banks are in the Mach 5 at the bottom of a lake and the Mach 5's oxygen tank has ran out. They begin gasping for air, but the way the VAs performed this scene it made it sound like... something else.
"The Fastest Car On Earth":
When Speed opens the picnic basket to see that his lunch is missing, he shakes it up and down to see if anything comes out.
Pops splashing buckets of water on Speed to fix his coordination which was negatively affected by the high speed of the GRX. Then when Trixie pushes Speed out of the way urging that he's had enough, she gets splashed too. Cue a dripping wet Speed laughing his ass off.
Speed, suffering from the after-effects of v-gas, is now terrified of even going remotely fast (60mph). How does Pops go about curing him? By tying Speed up and showing him a movie of going down the road from a first-person perspective, and Speed is still terrified despite not even moving at all.
During the early parts of the Casa Cristo sequence, Speed and Racer X are duking it out with a set of headhunters. They're doing all sorts of flips and knocks and standard Speed Racer car stunts. Then one of the headhunters goes for one of the other drivers and Racer X flips his car to go after him... and takes him out by punching the driver in the face. In a movie full of the cars doing all the fighting, this was particularly hilarious.
To fully appreciate this scene, you really have to see the Ninja's reaction to noticing the ring (at the same moment that he is put in the hold, and also the same moment that the audience gets to see the ring). His reaction is palpable.
Speed has stalled the Mach 6 during the final race. Pops manages to diagnose the problem and figure out a solution, then Ma Racer asks, "Would Speed know that?" Cue the Oh, Crap! looks on the Racer family.
Pops suspects Spritle and Chim-Chim are watching the race, as forbidden. They are, and quickly flip the channel to a nature show. In German.
Pops: It's in German.
Spritle: Not the monkey parts.
Which is ironic, because the film was shot in Germany.
Spritle and Chim-Chim are acting out their favorite anime on the couch when Ma Racer walks in:
Ma: What are you two doing?
Ma: Is this the same nothing that destroyed my last couch?
During the Casa Cristo, at the point where Speed and Trixie are arguing, Racer X cuts in with a "Children, focus!" that sounds exactly like an older brother telling off his little brother. It's funny every single time.
After the heart-to-heart between father and son, and Togokhan bringing the race invitation, Pops finding out that Sparky had been in the kitchen listening in.
Pops: What were you doing in there!? Sparky: Same thing as everybody else. [Cue Mom, Trixie, Spritle and Chim-Chim sheepishly coming out of hiding too.]
Just before Speed and Trixie kiss in victory lane, the movie pauses as Spritle and Chim-Chim, dressed as doctors, pop up to warn the audience of the disgustingness of the action about to take place.