Ragnarous :
Will it cause any damage to the motherboard if it's powered on and i plug in usb devices such as keyboard or mouse?
I've read somewhere that a guy plugged in a USB hub and it killed his motherboard
Do you mean on the motherboard's I/O? If so, yes. If you mean to connect it to the motherboard onto one of the USB headers, then no.
Oh... Now i got what you said

Yes of course you shouldn't mess with the motherboards organs in any way when its powered on !
But has it ever occurred to you, that you pay 300-400$ for something that can soooo easily be damaged? I mean they will improve its toughness if they are gonna sell it for that much money but nooo....
- Sam.