Sooooo you think you will never need a spare huh!!! Here's my story...
Back in the early 90s I had an '84 Camaro. If you're familiar with them the rear window was part of the trunk / hatchback. Rear window is similar to the 64-66 Barracuda window. Not as big but concave in nature. One day it was parked in the driveway, trunk facing the garage door and I hear this loud boom. I go outside and there's glass all over the ground and very little glass inside the car. My neighbor just happened to be mowing his lawn towards the direction of my car and he saw the whole thing happen and stated nothing hit it, it just exploded for no reason. This scene looked like this... all the glass was going. At least 90% of it was on the ground and very little in the car. The rear spoiler which is connected to the trunk hatch was sticking out one inch. This lead me to believe that the concave nature of the window and the extreme summer heat buildup inside caused it to explode. This caused the glass to go flying outward. No other explanation. Best way to envision it was if someone was laying in the rear compartment area and kicked the glass outward. So I always said I would get an extra one (and I did for $50.) just in case the same thing happened to my '66 Barracuda. From that day on, I always crack a window at least a half inch in the summer time..... anyway I have it stored upright in Storage with padding and boxes surrounding it.