Biggest misconception ppl have with modular connectors is that they all line up the same. If that 8pin modular was top row 1-2-3-4 12v and bottom row 5-6-7-8 ground, on a eps cpu 8pin connector you'd see 1-2-3-4 top and 5-6-7-8 bottom. But on a pcie gpu 6+2pin connector, top is 5-6-7-8 and bottom is 1-2-3-7.
If you'd successfully plugged that gpu in, for all intents and purposes you'd have plugged 12v to the gpu ground, which is attached to the motherboard ground, cpu ground, ram ground etc and depending on the reliability of the short circuit protection in the psu, could have pretty much done anything from no damage to completely frying the entire pc, if not starting a fire.
Corsair psus have decent set protections, even on the budget class psus, so chances are good of no immediate damage, but there's always a black sheep somewhere in the bunch so chances are just as likely you'd have a dead pc.