Back in 1997 at the height of Dexter's Lab's fame, an episode called "Rude Removal" was created. The episode was quickly shelved because of it's content which included copious amounts of swearing, even after being censored it was still shelved; in 1998 the lost episode had been seen at a small amount of conventions but since then it was lost completely and many questioned it's existence until last year when Craig McCracken reported that it indeed existed in completed form which was later confirmed by the show's creator.
Late last year, Adult Swim showed interest in the episode and wanted to air it on their channel but it wasn't until January this year that the episode actually aired on Youtube on Adult Swim's channel but was sadly made into a private video so I can't show the episode here. (Sorry there are rules with this sort of thing) but the episode has been re-posted numerous times and should be easy to find so happy hunting.

The episode starts off as normal with Dee Dee annoying Dexter while he's working on a device called the Rude Removal Machine, a fight breaks out and the two accidentally make rude clones of themselves while the originals revert to British Accents and act all friendly and polite.
This is where the infamy comes in as the Rude Clones do nothing but swear and curse which later involves Dexter's Mom as they insult her constantly before returning to the lab to be stopped in their place; after succeeding in removing the clones the episode ends with Dexter's Mom preparing the soap for washing out Dexter & Dee Dee's mouths and so ends the lost episode of Dexter's Lab.
The language in this short is real, while censored it's still very, very real.
It's not uncommon for cartoon makers to go off the boil and make something too rude for broadcast in order to off load some of the pressures of animating.
Even as early as the 40s where many popular characters such as Bugs Bunny & Mickey Mouse were making war propaganda cartoons and racial stereotyping to match the mind set of the era. These days a mostly squeaky clean cartoon like Dexter's Lab would revert to a swearing episode just to offload stress.