What are the most common mistakes people make when creating a mobile app?

Question

Android

I was interested in the question, what mistakes happen when creating mobile applications - site versions for example?
Usually always at the beginning - when you communicate with developers, for example, on website development - everything seems so rosy, everything is described well! as soon as they start working, bugs come out and so on, it happens that you have to turn to other developers and finish it off.
Tell me, is it the same in mob applications? How to avoid it? what to look for, even when looking for the appropriate developer.
Since with limited resources, it will not always be possible to finish constantly. I want to do, cut a little and roll out.
but you even look at popular sites - people complain about applications - what slows down, it's not right, it's not. Although there, I'm sure, far from stupid people are sitting there, who washed down this application, and they had a considerable budget.