I'm amazed that I've been writing and creating since I was about nine and I've made less then five of these mistakes... But anyway...
Remember people, when you're running low on ideas for characters, make a talking cat with a pink lion mane named DJ or something. Remember that cute, cuddly pets are wonderful for comic relief and just might come in handy for stealing fish or something. Your readers will never get tired of the cute, comic relief pet that is completely pointless and will never, ever want to punt it into the sun.
Stay away from words like bicycle, or asylum. People won't care if your narrator refers to them as "bikes" or "crazy houses" when in serious context. The information of such things is too hard to find for you. But the Cheez-its aren't, so eat those and don't care about anything how much you need to improve.
Mistakes are below your characters. No matter how clumsy your character is, carrying heavily things that go over pile high over their heads, they will never fall. Ever. Stumble maybe, but gravity works perfectly with them and God's graceful hand will catch them or some crap that I'm not sure what I'm saying anymore.
Also, normal animals do not exist. There are only magical animals that slew all of the house cats and gold fish in the great war that you're too lazy to write about. The only kind of animals that are out their now are hybrids. And if they look too much like a single animal from before, they can talk. They can talk because they are the last of their kind.
(I know, it's not as good as my first post on the bottom of page 14... But it's something. To make things horrible, remember that the animals, the comic relief pet, and the pile-of-heavy-things-that will-never-make-the-chacter-fall all use kung-fu. So write battles about them. Write many battles about how your characters kick the crap out of those evil-pile-of-books named Xavadaperli, the Evil King of Urinal Cakes in Book-form!)