For your application......2005 ZX6R 636......CR9E (stock plug) AND CR9IEX (iridium alternative) show identical reach specs (19mm) per the actual NGK website. In theory, should be perfectly interchangeable and usable.
A good spark plug replace habit to get into is to line up what is coming out next to what is going in....do a last chance visual comparo to eliminate any error whatsoever....part number, packaging...any conceivable error.
So, you tell us...exactly what plugs *came out*? Did you compare? What went wrong?
'95 Ducati 900SS/SP
'07 Ducati SS800
'98 Honda VFR800 FI
'19 Honda CBR650R