don't ever pair your gpu on your motherboard. while doing the mining process, all parts of your gpu will increase its temperature and it will melt the pcie socket on your motherboard.
Has this ever happened before? First time I am hearing about this possibility...
I have a pair of GPUs on the mobo and they are alright so far.
Nope, that never happens unless some parts are defective. All motherboards with 2+ pci-e full size slots are obviously designed to run with 2 (or even 3) cards installed directly into them (with SLi/Crossfire use in mind). The problems with connecting multiple GPUs to the motherboard directly usually start when you go overboard, like when you manage to connect 4 power hungry cards, for example. And it's not the PCI-s slots that melt, it's the 24-pin power connector. Used to be a common problem back in the day when ribbon non-powered risers were used, I've managed to burn one of my boards' 24-pin connector myself.