Blender Guru Tutorial - Part 5 - Coffee Cup I'm following BlenderGuru's tutorial Blender Beginner Tutorial - Part 5: Modeling

I'm trying to model a coffee cup. I started with a reference picture of the cup, added a cylinder, resized the base of the cylinder to match the cup, and resized the bottom of the cylinder to make the shape tapered.

However, when I add a loop cut to the middle of the cylinder mesh and scale that loop, the bottom edge loop becomes distorted and jagged for reasons I do not understand; it is no longer the nice ring that I want it to be.  



What's going on?
You have Proportional Editing on with Random Falloff selected. You can turn this off by pressing O once or twice or by changing the menu manually at the bottom of the 3D Viewport.

A Gif to explain:



Proportional Editing can be extremely useful for deforming a mesh in a mathematically precise way. For example, with your coffee cup, I might loop cut the main cylinder several times and then use proportional editing with Smooth Falloff while scaling the bottom loop to create a loose conic shape. I then might use Sphere Falloff to widen the middle and shrink the top.