When you build a responsive site, you have media queries. As long as the screen is more or less the size of common media queries, for example Bootstrap's defaults, a design will probably look as expected. However, if you resize the browser window, chances are the design won't collapse properly. This is a very common issue with navigation menus.
Now, my question: is the amount of users who resize their browsers really representative? And if so, what happens in terms of usability if I do not adjust the sizes for all possible sizes in which a user can change the size of the browser window??