Artistic License: Two particularly outstanding examples to anyone who has ever been on a motorcycle or done any research about how they work. Real-world motorcycles have what is called "counter-steering", where riders turn their handlebars the same direction (i.e., pushing the right handlebar forward while turning right) in order to help them lean better and turn smoother. Bikers in GTA lean, but they turn their handlebars the way you would turn a car's steering wheel.
The second example involves Johnny's cell phone. While no one in their right mind would talk on a cell phone while riding except on a Bluetooth, the problem comes from the fact that all bikers in the GTA universe hold the phone in their right hand. On motorcycles, the right hand controls the throttle, which means no biker could ever accelerate whilst talking on the phone.