"YouTube is the second-most visited website in the world, and its algorithm drives 70% of watch time on the platform—an estimated 700 million hours every single day. For years, that recommendation algorithm has helped spread health misinformation, political disinformation, hateful diatribes, and other regrettable content to people around the globe. YouTube’s enormous influence means these films reach a huge audience, having a deep impact on countless lives, from radicalization to polarization [...] 37,380 YouTube users stepped up as YouTube watch dogs, volunteering data about the regrettable experiences they have on YouTube for Mozilla researchers to carefully analyze. As a result, Mozilla gained insight into a pool of YouTube's tightly-held data in the largest-ever crowdsourced investigation into YouTube's algorithm. Collectively, these volunteers flagged 3,362 regrettable videos, coming from 91 countries, between July 2020 and May 2021. This report highlights what we learned from our RegretsReporter research. Specifically, we uncovered three main findings: 1. YouTube Regrets are disparate and disturbing. Our volunteers reported everything from Covid fear-mongering to political misinformation to wildly inappropriate "children's" cartoons. The most frequent Regret categories are misinformation, violent or graphic content, hate speech, and spam/scams. 2. The algorithm is the problem. 71% of all Regret reports came from videos recommended to our volunteers by YouTube's automatic recommendation system. Further, recommended videos were 40% more likely to be reported by our volunteers than videos that they searched for. And in several cases, YouTube recommended videos that actually violate their own Community Guidelines and/or were unrelated to previous videos watched. 3. Non-English speakers are hit the hardest. The rate of YouTube Regrets is 60% higher in countries that do not have English as a primary language (with Brazil, Germany and France being particularly high), and pandemic-related Regrets were especially prevalent in non-English languages." (Executive summary)