Too many areas:
* Executive-level hiring. How many startup founders bring entire crews of executives that are recommended by VCs. Those transplanted execs then bring in their own "crew", and proceed to not only change the culture, but also repeat many of the same mistakes, pursue the same processes, repeat the same groupthink that has been done before. It reminds me of the exec hired in HBO's Silicon Valley who developed the "box" and brought in those sales and VPs.
* Processes. How many startup founders blindly adopt OKRs, growth strategies, and methods without even thinking if they are relevant to their industry, business, culture, or skills?
* Technologies. VCs push (forcefully sometimes) the use of technology by their other portfolio companies, leading to lots of misfit applications. There's too much cloud-centric thinking which causes problems. The cloud is great, but its not a one-size fits all. Some independent thinking is helpful. Not everything needs to be in the cloud
* Grow grow grow, then fire fire fire. The grow-fast then fire-fast approach mentality is really harmful.
* Following VC recommendations for core business operations, such as who to hire for law firms, accounting, HR, and other operations. Is it a wonder that so many startups make the same mistakes, do the same things, and run the same processes when they are all using the same vendors? Not too long ago, banking with SVB was "just a thing we all did". Look where that group think gets you. Maybe you don't need that law firm, accounting firm, or HR just because every other startup you know is using them.
* Idolization. There's too much worship of enterpreneurs, investors, etc. Yeah some of these folks have done some great things. But don't take every word they say as if it's gospel. It's ok to disagree with PG, Elon Musk, Peter Thiel, YC, etc. etc. whomever you idolize. They're not always right, and perhaps they're often wrong. Groupthink often happens when people exhibit cult-like behavior by following the herd or guru.
* Diversify your sources of information. Don't read so much HN or TechCrunch or whatever without balancing that information diet with other sources that aren't part of the same sphere of thinking. Groupthink happens in echo chambers and information bubbles. HN is one.