Really this should not have been possible as the PCIe 8-pin connector was designed to have a completely different arrangement of pin shapes than the existing EPS12V connector. These are shown in the image below (notice the difference in bevelling on the corners):
(Image from here)
It appears from the picture you posted that the cable that when you pushed it in, the corners of the square pins got scraped away allowing insertion - the moral is don't force connectors in (I'm not trying to be condescending, but it's worth keeping in mind).
In this situation you have probably been saved by virtue of the card in question having both a PCIe 6-pin and a PCIe 8-pin connector. By having had both the 6-pin connector plugged in at the same time as the EPS12V connector, from the diagram above it is clear that basically all the 12V's and all the GND's are shorted together. While not a very good thing at all, it means that the PSU will not have been able to turn on, nor apply any voltage on the 12V lines.
Essentially this means your graphics card will never have seen any damaging voltages from having the polarity reversed on one of its connectors. So in theory the card should be absolutely fine.
For reference, if the card has just an 8-pin connector, then if you powered it with an EPS12V, it would have been exposed to -12V which would have almost certainly have caused it to fry - and would probably also have taken the motherboard out along with it!
As a side note, it would actually be possible to easily plug an 6+2-pin PCIe connector (these are the ones that can be separated or combined to be either 6 or 8 pin) into an EPS12V socket - in fact I did this once, but fortunately realised before turning it on. The true 8-Pin connectors (the ones which cannot be split off to go into a 6-pin connector) have a flat bar which joins across two of the pins to prevent doing this.
I wasn't aware that it is possible to do that - by design the EPS12V cable has a different arrangement of notches to a PCIE 8Pin cable which should prevent insertion. – Tom Carpenter – 2015-11-14T22:55:15.563
It seemed to fit, I'll admit it didn't fit as easily as the PIC-E 6+2 which just dropped it, but it fit. Hopefully it works when I get a chance to test – chiloutus – 2015-11-14T22:59:58.513
I presume you powered it on. Did the PSU just shut down immediately or keep going? Also could you post a picture of the end of the EPS cable, I'm quite curious to see how it could have gone in the socket. – Tom Carpenter – 2015-11-14T23:02:54.183
The PSU didn't seem to power on, no fans or anything like that spun up. Also I will upload a picture now – chiloutus – 2015-11-14T23:15:36.720
It seems as you pushed the connector in, it stripped the corners of the square pins (not very good plastic!). If the PSU didn't power on, it means it was clever enough to detect that you had basically shorted 12V to GND and not turn on (the 12V on the EPS connector would short out with the GND on the PCIe 6-pin connector and vice versa). The card is probably fine as it will never have seen any voltage. – Tom Carpenter – 2015-11-14T23:20:47.573
Pretty concise, okay I will see what I can do tomorrow about getting somewhere to test the card and update. Thanks! – chiloutus – 2015-11-14T23:23:14.727