Some that I can think on the top of my head:
1) Assumptions: Nothing kills a business faster than assumptions. It is easy to assume about a pain/problem that may not be a big deal for others. In several cases, it can be that the problem just doesn't hurt enough to consider paying for it. It is always best to test your hypothesis by asking the right questions: Mom Test and performing validation: Lean Startup.
2) Working in Silo: Starting up is hard. It is a lonely path that is not taken by many in society. Working alone, without any comrades can also become depressing. However, due to the presence of communities like Indie Hacker, it can easy to seek feedback and learn from other entrepreneurs and they should be the staple of every entrepreneur.
3) Not caring about your customers: If you are not empathetic to your customers, then they are highly unlikely to become your loyal fans. In the end, you only need a 1000 or even just a 100 true fans to quit your 9 to 5 grind.
4) Not marketing your product: Build it and they will come simply doesn't work. You must have heard that a few dozen times, but, it is simply difficult for some creators to internalize. As a creative individual, we just want to create and want our art to be appreciated. This is far from the truth. You need to get the biggest megaphone and talk about our product. If you are truly passionate about it, spread it like a virus. Passion is often contagious.
5) Undervaluing your services or product: If you undervalue your product, people are often going to consider it as low quality. Often you will lose several customers along the way. However, you will end up find a customer segment that isn't too price-sensitive or understands the worth of your creation. This also results in fewer returns and you will have to make fewer sales to be able to reach the same revenue. Always test your pricing and ensure you provide as much value as you can to justify the price.
6) Blindly copying your competitors: Sometimes your competitors are stuck in old practices of the industry and just adopting them mindlessly will just increase your overhead and keep you stuck in price wars. Create Blue Oceans by eliminating these irrelevant practices and increasing value in say customer support or making your product minimalistic or simple to use. Plus, if you are always copying your competitors, then you will always be a few steps behind. Innovate and lead the way for your industry.

7) Getting comfortable(complacent): As several of us, are side hustlers, if you get too comfortable with your job, then you are unlikely to break free. You need to have some discomfort or fire to keep hustling for your dreams.
I don't have any affiliation with any authors for books that I have posted. Unfortunately, I am unable to post the links to the books in the post. I will be adding them to the comments section I believe those are really good books that can elevate your Indie Game. This post is more like a reminder for myself to not forget about these points. Hopefully, you will benefit from it too.
Do share some of your thoughts and points that I may have missed and also if you disagree with any of them. Have a great day ahead and stay Indie ✌
Oh burning out is a big one and often ignored.