I was trying to buy a movie but I hit rent by mistake. Now the movie is downloading. What do I do
When the rental period ends, you will have to go to the movies page in the iTunes store and then purchase it. iTunes purchases are final including rentals.
I believe the iTunes Store can make a one-time exception to this rule and refund the item for you. Just send them an email explaining your situation, and expect a response in 24-48 hours. Visit http://expresslane.apple.com and fill out the email form.
Meanwhile, you might want to make sure your iTunes Store warnings are turned on, so you can avoid making purchases by mistake. Look here:
Resetting iTunes Store warnings
I wanted to buy a movies but hit rent by mistake. What do I do
