The Bandwagon fallacy (argumentum ad populum); concluding that something is true, good, or correct simply because many people believe or do it. Sounds obvious when you say it out loud, and yet it's probably one of the most common errors in how people actually think day to day.
For most of human history, going along with the group was genuinely useful. If everyone in the village agreed that a certain fruit was poisonous, you didn't need to test it yourself; following the crowd literally kept you alive.
This instinct still runs in the background, except now instead of fruits, it's deciding what news to trust, which opinions feel valid, what products to buy, which ideas are worth taking seriously.
The crowd is not always wrong, but it is never right simply because it is a crowd.
Once again, to be that guy that always complains about social media, social media made all of this significantly worse. Now everyone has a platform to just yap, the volume of the voices are through the roof, but the quality didn't scale with it. What we got instead is some farmed confirmation bias with likes, followers, and reposts counts. A post with 50k likes feells more valid than one with 15, even if the post with 15 likes is more accurate. Nobody decided to think this way, it just happened.
The AI trend is also worth paying attention to, it's basically a newer version of the same problem. AI summaries are basically the average opinion on a subject, not the fundamental truth. When you ask AI a question, it drawing on patterns from massive amounts of human-generated publications on the internet, in practice, that often means it's giving you the most common answer, that's what it ranks as truth. Which sounds reasonable until you remember that the internet is basically full of people running the same bandwagon and confirmation bias loops we just described.
The average opinion on a topic and the correct answer to a topic are not the same thing. Sometimes they overlap, often they don't.
This is where the distinction between popularity and consensus actually matters. Concensus isn't just a lot of people agreeing, it's a lot of people who are qualified, working independently, looking at actual evidence, and arriving at the same conclusion despite all that. That's meaningful. The convergence in spite of independence is what gives it weight. Popularity has no such requirement, it can be entirely created by repetition, algorithms, and basic human preference for not being the only one who believes something.
If you've been paying attention long enough, you'll notice that popularity and quality are often inversely correlated, not positively. The most-shared article is rarely the most accurate, the most-downloaded app isn't always the best, the loudest voice in the room is rarely the most informed. This isn't always true, but it's true often enough that it's worth holding in your head as a default suspicion rather than an afterthought.
None of this means you should distrust everything, that's its own trap. It just means the question is worth asking; not just "do a lot of people believe this?" but also "how did they arrive at this conclusion?" and more uncomfortably: How did you?