Every couple of months, we can reliably count on a big brand to help us learn a lesson about online reputation management. Today, we give thanks to P&G for today’s lesson:

The conversation starts when your customers say so, not you!

As AdAge reports, P&G has launched what it considers to be the most significant improvement in its Pampers line of diapers, in 25 years. Unfortunately, the company decided to start shipping the new diapers in old packaging, without explaining the benefits of the new pooper-protector.

Engineering this manufacturing overhaul meant putting the new diapers into the old packaging and into stores starting last summer in parts of the country. But because the new diapers had only reached a fraction of the U.S. by fall, P&G wasn’t ready to launch its campaign. Without marketing or communication, some consumers in early markets reacted strongly and spread the word virally to markets that did not yet have the diaper.

OK, who does that?

In this day and age of socially connected consumers; who launches a dramatically different product without also readying a TV, print, search, and social media campaign?

OK, maybe I’m being too harsh. Maybe P&G just didn’t realize that it was heading towards such a consumer backlash…

"Similar to our experience in the past, when you change things, without articulating what the change is about, you will get consumers who complain," said Jodi Allen, VP-North American baby care at P&G.

[Jon Stewart'esque pause]

What the…??? You knew you’d get this consumer backlash??? Then why didn’t you have a plan in place to explain the benefits of the new diaper. Why didn’t you have a Twitter channel to answer questions from concerned parents? Where was the Facebook FAQ page to explain how the product will improve the lives of parents–and their babies?

If you’d taken the time to do the above, Rosana Shah of Baton Rouge, La, wouldn’t have felt compelled to launch her own Facebook page to warn other moms…

"…they’ve slipped this inferior diaper into the existing packaging without notifying the consumer."

At this point you could argue that this Pampers backlash is just a small minority. After all, the "Bring Back the Old Cruisers" fan page on Facebook had only 20 members as of last week. But, I’d point to the fact that new parents trust the opinions of experienced parents AND this "backlash" has now reached a wider audience–thanks to AdAge, and now us.

Fortunately, P&G doesn’t plan to just sit back and let a few upset consumers have their way…

Ms. Allen believes the tone will change quickly once P&G turns on marketing support, which will include a first-ever, pre-launch buzz campaign starting this month.

OK, that’s all well and good Ms. Allen, but you should have had this in place the moment you shipped your first revolutionary diaper. Heck, two months before you shipped it!

So, thank you P&G. Thank you for being yet another mega-brand that, despite its millions in marketing budget–and expert marketers to boot–failed to realize one of the most important lessons in reputation management.

You don’t get to decide when the conversation about your new product starts. So be prepared from day one!