My first ever mistake was when I was in my mother's womb and I thought: "Let's go and see what's out there."
Another big mistake was when I didn't understand that instead of continuing to be an employee of a company that moreover paid me little (but, you know, the passion...) I should have chosen to work as a freelancer or at least change company.
Maybe I went off topic; strange, it never happens to me
The most common mistake is trying to save time when analyzing. Mine was slightly different: I remembered a game with love and developed it, but I was a child; only when it was completed did i realize that it wasn't that exciting so i decided to add a second game in the same app. Not having foreseen this, it is necessary to modify everything (many things), data structures (DB and in main memory), classes, communications, ...
So it's
like having made a mistake in the analysis phase, the usual biggest mistake.
For example, I currently have to revise a project for which the customer wanted only a few input data and a simple layout about a year ago.
If your customer changes his requests and wants something else, from then on make him pay double your normal rate.