In the episode "The Swarm" Rex and Bobo are competing to see who can kill the most bugs. Near the end of the episode Rex tells Bobo that he killed the rest of the bugs. So what does Bobo do in retaliation? In the very next scene when the reporter is explaining how Providence killed the bugs. After the scene as Rex turns to go to his room he waves at the crowd for a split second, but it suspiciously looks like he flipped everyone off.
Reporter: ... Lives saved in large part by something you might find hard to believe
Bobo: Wait for it...
Reporter: These colorful undies, (Everyone heckles Rex) our sources say that they gave Providence scientists the scent they needed to turn those bugs against themselves.
Rex: Why's my underwear on TV!? (Six and Holiday look at each other and turn around) Sources (Rex turns to Bobo)? This wouldn't have anything to do with the fact that I stomped more bugs than you... Would it?
In "Outpost," a woman in South America calls Rex "gringo," a racist term for a white person as described by someone from a Spanish-speaking country (particularly Mexico, as a lot of the Speedy Gonzales cartoons from the Golden Age Of Animation used "gringo" to describe Sylvester the Cat).
In "Double Vision" Rex says "Horse Manure" after the giant vines come out of the ground with a Oh Crap face on.
In the same episode a lot of things get squirted in Rex's face.
In Rock My World both Beverly and Rex get called by Holiday at the concert. She reminds them to "only drink the bottled water."
That might not be getting past the radar so much as something any kid old enough to go to a concert needs to know.