MacBook Pro retina 2012 vertical line I have this vertical line at the side of my MacBook Pro, any idea what could be causing this/how to fix it?
This looks like an issue with your display, your flex cable, or some thing in the graphics part of your motherboard (a bad gpu, bad gpu solder balls, or a bad tantalum capacitor for the gpu).

If you feel comfortable, reseat the video flex cable and see if that solves it. If you don't feel comfortable opening your laptop, you'll need to take it in for repair. 

Messing around inside a computer can always cause issues, even for experienced technicians, so if you feel uncomfortable doing it, don't risk it. You could permanently damage components.