Cross-Dressing Voices: Averted, all of the major male characters are done by males (save for Elmo, he's done by the same girl as Beebe), though Doug is done by female seiyuu (voice actor) Masako Nozawa in Japan. Played straight with Todd Bentley in "Doug's Fan Club," who was voiced by a young Melissa Greenspan (of Desperate Housewives and Providence fame).
Not quite averted: Doug's grandmother was voiced by Fred Newman, who also played Skeeter, Mr. Dink, Porkchop (yes, the dog), and others.
And then we've got Skeeter teaching kids how to read on Between the Lions. Oh, and he hosted Nickelodeon's early talk show Livewire, and famously wrote a book about how to become a Man of a Thousand Voices like him.
Keep Circulating the Tapes: While the Nick series has DVDs on Amazon, there's the good chance of playback errors, and two episodes are missing from season four because of "technical difficulties". However, there are torrents available of all four seasons of the Nick series. For years, this was played completely straight with the Disney series, with the videos out of print and only a handful of episodes online. Heck, The Movie wasn't even released on DVD (though it is available for video rental/purchase on Amazon). This was finally rectified in 2019 when the Disney+ app made the entire Disney series, plus the movie, available for streaming (legally).
iTunes has all four Nickelodeon seasons, and the two episodes missing from the Season 4 DVD have been reinstated.
The movie was released on VideoCD in many second world countries. The catch? was. It's out of print.
The Merch: Almost all of the Doug merchandise available was from the Disney series. Nickelodeon's merchandising was more or less limited to videos and smaller-ticket items like party favors, although there was a board game based on the Nick series.
No Export for You: Some foreign markets only air the Nickelodeon series of the show. Canadians got the Disney series in English on CTV, but only the Nick series got a French broadcast in Canada.
A few episodes were skipped in NHK BS2's Japanese airing of the Nick series.
The Other Darrin: Thomas McHugh and Chris Phillips replaced Billy West (who didn't want to be part of the Disney variation of Doug) as Doug and Roger's voices, respectively, in the Disney series. This was made note of with Doug in its first episode.
Talking to Himself: Doug shares his voice actor with Roger in the Nick series.
After Billy West left, Roger ended up talking with himself with Boomer's voice actor in the Disney series.
Judy shares her voice actress with her mom Theda.
What Could Have Been: The original Doug product was going to be a children's book, Doug Gets a New Pair of Shoes, until the creator found out that Nickelodeon was looking for cartoonists for new original cartoons (during this time, all their cartoons were from different companies), and decided to turn it into a cartoon series. The book ended up serving as the episode Doug's Cool Shoes in season one.
Roger was also going to be a rival to Doug for Doug's love interest, Patti. This was scrapped, and the role was taken by Guy once the show was switched over to Disney.
The movie was also going to be a Direct to Video release, but Disney decided put it into theaters due to the success of The Rugrats Movie. Critically, it didn't work. In box office numbers, it grossed much less than Rugrats, but it was still a huge profit for Disney.
The movie was also going to be made by 20th Century Fox in 1993, along with movies for Rugrats and The Ren and Stimpy Show. Nick's contract fell through before they could make a movie deal. So once Viacom (Nick's company) bought Paramount, Rugrats got its movie, Disney ended up making Doug's movie, and Ren and Stimpy never got one. It's unknown whether Fox's version of Doug's 1st Movie would've been the same movie as it became.
Also, the movie was originally going to be called The First Doug Movie Ever, but was later retitled to Doug's 1st Movie.