Should i be worried about your cat jumping of your apartment balcony?

It can happen by accident. Cats are really good about calculating their jumps to land on the sill or the rail, but they can make a mistake, and all it takes is one. A friend of mine lost a cat exactly that way: a miscalculated jump to the rail of a third-floor balcony. Just don't let your cat out to the balcony. My kitten miscalculated jump from my half-height bookshelf to the floor (which isn't even that high), and she jumped a little too far out and hit her head against entertainment center table... So they DO make mistakes especially if they haven't ever tried that particular jump space.