I went to fill up the other day and put in 9 gallons of 87 in my 03 Cobra before I realised. I switched at that point. I started out with a 1/4 of a tank. What will happen to the car?
You'll probably be fine, but be careful if you drive it WOT because you might get some detonation. I'd throw in a bottle of octane booster just for insurance.
I would just drive it a while and keep topping it off to keep diluting the 87 octane. Should not be a big issue though. Like above said, stay out of the rpms and since you got an 03 the boost too
03/04 Cobra's DO NOT have knock sensors! The SC makes too much noise so Ford didn't equip the new snakes with them.
Drive it around and stay completely out of boost until you've burned off enough to top it off with some 91 or higher again. As long as you're not flogging it and putting a heavy load on it you'll be okay. Don't bother putting ANY off-the-shelf octane booster in your car either, it would take 20 bottles just to raise the octane rating a whole point! Octane booster is a COMPLETE WASTE OF MONEY!