You cannot confess sins when you're about to die

I've seen lots of movies and shows where the character confesses all of his sins to the priest/nun before he's about to die. I grew up in a strict religious family where the church would often say that you cannot confess sins when you're about to die. You have to confess sins when you still have the chance. The priest won't forgive you if you confess while you're about to die.