Well the good news for the EPA, your catalytic converter is consuming those HC"s, but maybe not for you if you have to replace it.If you start to notice spark blowout during WOT it's time to take care of your plugs. I wasn't able to get to my spark plugs for three weeks and I noticed my daily commuting fuel economy dropped by about 10%. Even though I don't drive in WOT that much the plugs weren't igniting the fuel as well and I was leaving unburned fuel to go into my exhaust system.
Don't have to get nitpicky on the cause for a failed spark, blowby, excessive gap, excessive carbon on the center electrode insulator, poor connectivity, shorted turn in the ignition coil, poor connectivity to the ECU, water in the fuel, etc. All cause for a misfire, even dirty fuel injectors, or a dirty air filter.
Just tells you, its time to look at it. Check mine every 15K miles because I already know misfires will occur after this time. Always has, always will as long as we are pouring carbon in the fuel.