Some cats are either clumsy or not that bright. My cat Angel (who lives with my parents) tried 8 times in 3 minutes to jump into a closed window. It was the middle of winter and about 10 degrees F outside so there was no way I was opening it. She just couldn't figure out why when she jumped there she kept hitting something! I've also caught her running full force into walls and walking right off a table. And she can see fine! The vet gave her a clean bill of health and vision!