If I understand correctly, all modern jets would be impossible to fly without a computer...
The technical term for this is "dynamically unstable."
The difference between "stable" and "unstable" aircraft is just this: if you make no control inputs to a stable aircraft, it will fly in a nice, steady attitude. If you make no control inputs to an unstable one, it will lose its attitude almost immediately.
Here's your handy, do-it-yourself demonstration.
1. Make a paper airplane and throw it. A well-made plane will fly straight and level: it's stable.
2. Throw an unfolded piece of paper. It will flutter and flap and fall to the floor: it's unstable.
Modern fighter jets are pretty much all designed to be unstable. For one thing, it makes them much more agile. Second of all, it allows the designers to achieve some high-speed aerodynamics that they couldn't otherwise achieve.