What is the most common Startup mistake?

Shipping with bugs.

no marketing at all

Being in love with your solution more than the customer problem.

I would say that this is not a serious attitude to marketing. That is, the product is made for the sake of the product, and not for the client.

Not focusing on Churn and customer retention above all else.

I think ignoring customer feedback is the common situation that start-ups may encounter in the 0-1 phase. User feedback is the most important thing, but some founders focus too much on the financial part.