The following parts are easy to break during back glass replacement.
Soft Spots
Green Circle = Flashlight flex
Purple = Motherboard
Blue = Battery
Red = Charging Port Flex
Be very careful working on top of these parts, or else it will probably make your life harder.
Spongy Spot
Don't push a glass breaker onto this part of the back.
It will push the sponge under the motherboard here, and bend the board.