Not doing enough research and starting to build random product and solving the tech problem first.
After that I did more research and found better patterns with validation and brining tech at a later stage after building waitlist, validating and better research.
Built multiple profitable products in the last 1-2 years. Can get more here around my products https://twitter.com/upen946
Mostly school and not having enough money.
I did some ecommerce in high school, and being my first venture, it failed once I ran out of money and exams rolled around.
I then did some web design. This was far less cost intensive, since I did mostly cold calling for marketing. However, school got in the way again, and I eventually burned myself out on cold calling.
I find that if I dig deeper, for the second one, school was more of an excuse. It was fully possible for me to juggle both at the same time if I manage my time well. Furthermore, cold calling wasn't that effective anyways.
So I learned the importance of time management and working smarter, not harder. Now, I'm on my third venture with a co-founder while in university learning from my past mistakes.
How about you?