Plugs Dilemna

WHEN my 2004 Commodore SV6 had its 90,000km service, the dealer's report claimed the spark plugs were changed, but a few weeks ago the car developed a misfire under load. I suspected injectors and put a shot of cleaner through, but to no avail. So I took it to an Ultratune Service Centre where it was found that the spark plugs needed to be replaced. It now has a new set of plugs and is fine, and I'm $150 out of pocket for diagnosis and labour. In fairness to the Holden dealer, there was no charge for the plugs at the service. The dealer told me my service report was incorrect and that the Holden Service Schedule does not require plug replacement until 120,000 km. I intend raising the matter with Holden, but what conclusions can be drawn from this experience? First, always try to verify the work documented, and asking for the old parts might be a good idea. Second, Holden might have the plug replacement schedule for the V6 Alloytec 190 engine wrong.