Assuming your mom is a designer, working with Photoshop on her fairly new Macbook 15", and putting out comps for "mobile"*.

Somehow many of us assumed that an iPhone has a "screensize" of 320 x 480px, an iPad has 768 x 1024px and next up in line we have the laptop screens and desktop screens. Mom's Macbook for example has 1440 x 900px.
So, what's your mom gonna do? Sitting on her 1440 x 900px screen, she is very likely to open a photoshop document and give the canvas a dimension of 320 x 480px, and happily starts designing a screen that should fit in there.

*) Now for the sake of this article, I won't go into details of how working with a defined height/width canvas in itself is a problem for "mobile" design, there's plenty of info around that out on the web. Plus I won't go into details why with the arrival of the iPhone 5 and its 640x1136 screen the "this is iPhone" thinking is over.