mate of mine had this happen in his stagea too.
melted the electrodes right off 2 or 3 of the plugs.
not sure why, but once he replaced them & did a few other mods to his car (which were on the list anyway - fmic/adjustable cam gears/ebc/etc) he hasnt had any issues since.
i wouldnt be overly worried about it, unless it keeps happening, then you may have to go to a hotter(colder?) plug to make sure they can handle the power the engine is producing.
also to answer whoever it was that asked if the '11' was the heat range - its not, its the gap they are set too.
if you buy -11 plugs, they need to be regapped to at least .8mm, if not .7mm.