Faulty keyboard blown motherboard fuse?

I'm not sure what happened here but I just tried plugging in a second hand usb keyboard (A1243 model) and 'b' 'g' and 't' (and occasionally 6) just kept typing in an endless bgtbgtbgtbgt... until i heard (and felt) a sharp 'pop' from just underneath the trackpad. My macbook pro seems to be running fine still, the usb keyboard isn't doing anything now if plugged in and a hardware test returned no faults. Other USB devices seem to work fine. Was I just imagining this 'pop' or is it possible that I've blown something on the motherboard??

also could this bgtbgtbgt... thing have been caused by cleaning the keyboard with a damp cloth?

any ideas or info of similar occurences when pluggin in peripherals would be useful

thanks!