Let's take it logically. There are dozens of these things all over the world. Plenty of people witness the finished product so you would think at least one would be witnessed under construction. It would take dozens of people weeks to accomplish. Look how long it takes a Hollywood movie crew to construct something this complicated.
But there’s nothing else that makes sense. If it were highly-advanced UFO’s from another planet, they wouldn’t make the obvious errors you see in many of them.
First, as I said before I don't have to have an explanation just because a good one doesn't come to mind. Secondly, on what basis do you say that aliens don't make mistakes? Thirdly, I didn't see any mistakes in the one you posted above. I also didn't see any sign that of where a large group of people entered and left the field. Or made any misstep outside the design which would stick out like a sore thumb and be impossible to repair. Once you crush a plant it never looks the same.
Some have been observed being formed with spheres of light flitting about within them.
I don’t believe the circles are created by UFO’s, if UFO’s are understood as spaceships from some other planet. If UFO’s and crop circles are indeed caused by non-human entities, I think they are more akin to occult phenomena than anything else.
I’m tellin’ yuh, it’s the Good People!