We love these series. Albert Barillé's Once Upon a Time… cartoons taught a whole generation how the body works, where we came from and who went where first, and they still hold up as some of the best educational television ever made.
They were also made between 1978 and 1996. Science has moved on, history has been rewritten in places, and a few things were simply wrong even then. This site is not a takedown. It is a companion: go ahead and watch the episodes (they are free on the official Hello Maestro YouTube channel), with your kids or on your own, and come here if you want to know which lines to take with a grain of salt.
152episodes checked18outdated53common beliefs then61wrong in their day
Counting only major errors, the ones that would leave you with a wrong idea. Another 667 minor slips (a date, a name or a number slightly off) are listed on each series page if you want them.
Every point quotes the line from the episode, gives a timestamp, and says what we know today. Points come in three kinds, because "it's wrong" means very different things for a cartoon from 1978:
Outdated
Matched the science or scholarship of the day; later discoveries changed it. Nobody could have done better at the time.
Common belief then
Specialists already knew better, but encyclopedias, school books and popular books still said it, so a writer would easily have repeated it. Each comes with a note on why it was believed.
Wrong in its day
A careful writer could have got it right from ordinary references. Many of these are small slips in a date, a number or a name, and some are probably dubbing or translation errors.
Every point is also rated major or minor. Major means a viewer who believed the line would come away with a meaningfully wrong idea: a myth told as history, a wrong explanation of how the body or nature works, the wrong person credited, a people described as extinct who are still here. Minor means the lesson of the scene survives: a date or a number slightly off, a mixed-up name of a secondary figure, a likely dubbing slip. The series pages show major errors by default; minor slips are one click away.
Simplifications made for children ("the heart is a pump") are not counted as errors. They are listed under each episode as "simplified but fine". Each point also carries a confidence level, high or medium.
Methodology
Sources. We used the official uploads on the Hello Maestro YouTube channels, in the English dub. Where an episode is only available in the original French, it was checked in French and quoted in French with an English translation. A few episodes could not be found in English or French and are left out; each series page says which.
Transcription. Every episode was transcribed with OpenAI's open-source Whisper speech recognition model (large-v3-turbo, run locally through whisper.cpp with voice activity detection). The result is a timed transcript of the whole episode. The raw transcripts are published in the GitHub repository, unedited, so you can check any quote in context.
Fact-checking. Each transcript was read in full by an AI model (Anthropic's Claude), a few episodes at a time, with instructions to check every claim the episode presents as fact (dates, names, numbers, how things work, who did what first), to sort each problem into the categories above, to say what is known today, to be conservative, and to treat garbled names or numbers as likely transcription errors rather than the show's fault. The checker used web searches to confirm specific dates, figures and recent findings where needed. For Space, only real science and history were checked, not the science-fiction plot.
Quotes and timestamps. Every quote had to be copied word for word from the transcript, and was then matched back to the subtitle timing to get the minute and second it is said.
Review. We read through the results and corrected problems we found along the way. Examples include mislabelled uploads (an episode that turned out to be a different one) and old British "billions" (a million millions) that had been flagged as wrong when they were right for the time. This was a careful read, not formal peer review.
Limitations
The checks were produced with AI assistance and spot-checked, not written by historians or scientists. They can contain mistakes, especially points marked medium confidence.
Speech recognition mishears names and numbers. Where a quote looks garbled, the show may well have said it correctly.
We checked the English dub. Some "wrong" points are probably translation errors that are not in the French original.
"Today" means October 2026. Science and history keep moving.
Spotted a mistake? Please open an issue on GitHub with the episode and the point, ideally with a source. The per-episode reports the site is built from are in the repository as plain Markdown.