What are the most common beginner mistakes when learning Blender? I haven't found a question like this after a few searches. A lot of questions that come in are from beginners who are learning on their own. I know from experience what that is like, and how long it is before the sheer complexity and scope of Blender stop being very confusing. I don't know how many times I searched arduously to figure out how to undo something that was the result of one mistaken keystroke or click, or how often things have had to be rebuilt after an unnoticed mistake caused bad geometry.

I believe the most common mistakes are quantifiable and not a matter of opinion. I think it would be a real service to have the most common ones explained to beginners so they can be aware of them from the outset. I don't know of anywhere with such a guide. Maybe there is something like this here already and I didn't find it, if not, I hope this is considered not too broad or opinion-based.
Double Vertices - Most often caused by extruding something, cancelling the move afterwards, and then extruding again. Can be fixed by selecting all by pressing A, then pressing Ctrl + V, and choosing 'Remove Doubles'.
Flipped Normals - This can have many causes. It's visible as odd shading on a model, but is often a bit hard to pick out. Can be fixed by selecting all, then pressing Ctrl + N.
Unapplied Rotation and Scale - If the scale of an object is not 1 or its rotation is not 0 degrees on all axes, a lot of actions can have strange results. Unless there is a specific reason not to apply rotation or scale, can be fixed by pressing Ctrl + A and selecting the appropriate option.
Hidden things - Pressing H makes whatever is selected invisible. Pressing Alt + H unhides things. 
Unnessesary Ngons - Faces with more than 4 connected vertices (Ngons) may cause shading issues, and should be minimized. Ctrl + T triangulates the mesh, and can help avoid these problems.