It’s that time of year again when product marketers renew their annual fight for the attention (and spending) of a holiday-smitten public. One of the most effective ways of garnering that attention is word-of-mouth marketing – particularly when the “words” come from a celebrity endorser.

To even the most cynical among us, the celebrity culture is alluring. And, make no mistake: it’s a profitable endeavour. The stars of film, stage, television, music, sports and business not only capture our attention, they influence when and where our discretionary income is spent.

Overt product endorsements by pop-culture icons, or even something as seemingly innocuous as their wearing a certain designer or carrying a particular bag, can generate enormous public attention and translate into huge product sales. And what product manufacturer wouldn’t want the glamour, sex appeal and popularity of a celebrity to burnish their brand?

For many product marketers, Oprah Winfrey is the quintessential product endorser. Besides her celebrity renown and squeaky clean reputation, Winfrey has cultivated the trust and respect of devotees and casual observers alike. And if you have a product or book to sell, being included on her “O” or “Favorite Things” lists, or singled out as recommended reading by her “Book Club,” is akin to finding the Holy Grail.

Brands like Spanx, Uggs and Amazon’s Kindle, among others, have reaped significant benefits from being named to one of Winfrey’s lists. Even a passing mention by Winfrey can be sales gold: When Winfrey let on that she uses a LightWedge book light, daily sales of the literary illuminating product increased to $90,000 from $3,700 in a single afternoon.

That’s not to say that there aren’t risks associated with celebrity endorsers, however. A hard-partying starlet, cheating athlete or philandering rock star can draw the wrong sort of attention to a brand. High-profile companies including Nike, Champion, H&M, Nivea, Coca-Cola and even Agent Provocateur have found out this the hard way, and been forced to drop their spokespersons due to unsavoury activities. Tiger Woods, Kate Moss, Rihanna, Lance Armstrong and myriad others celebrities can surely attest to this phenomena.

Even non-paid endorsers can draw unwanted attention to a brand. Consider the actions taken by Abercrombie & Fitch (A&F), after a member of The Jersey Shore cast was shown wearing the brand’s clothes during filming in Italy. Worried that the individual’s eye-raising behaviour might cast a shadow over the brand, A&F took pre-emptive measures to ensure its brand integrity would remain intact. The company issued a biting statement that noted, in part, the association was “contrary to the aspirational nature of our brand, and may be distressing to many of our fans.”

But today, there is something else that can wreck a celebrity endorsement as surely and quickly as the most unseemly of behaviours can: inauthenticity.

In recent years, both Coca-Cola and Pepsi have come away red-faced when their paid celebrity endorsers, which included Brazilian soccer icon Ronaldinho and pop superstar Britney Spears, were caught enjoying the competition’s fizzy drinks.

Even the most trusted celebrity endorser – none other than Winfrey herself – can bring unwanted attention to a brand by behaving in a way that consumers find inauthentic. Recently, Winfrey found herself in a dust-up over a Tweet endorsing Microsoft’s new Windows-based tablet computer, Surface.

“Gotta say love that SURFACE! Have bought 12 already for Christmas gifts.#FavoriteThings.”

The problem? Winfrey sent the Tweet from her Apple iPad … one of her favourite things back in 2010.

One likely explanation is that Winfrey’s favourite things are not her favourite things at all, but little more than paid-product placements. This would certainly create some doubt as to the authenticity of Winfrey’s recommendations, which until now she had seemingly endorsed without compensation, assuming a company was willing to give its products away free to her audience.

In this regard, a 2011 Businessweek article now seems prescient. Calling into question the veracity of Winfrey’s product endorsements, the article went so far as to describe the shows announcing Winfrey’s Favourite Things as “tantamount to infomercials, though far more effective.”

While one well-reported Tweet from Winfrey is unlikely to affect Surface’s reputation or sales in an adverse way, or even linger as a smirch on Winfrey’s own reputation, it does raise an ugly spectre in the current age of heightened transparency and authenticity. If nothing else, it serves as an important reminder to public relations professionals that no celebrity endorsement – not even by Oprah Winfrey herself – is infallible.

While the rewards of a celebrity endorsement may be high, the risks to a brand’s reputation are often higher. The lesson: Authenticity rules.

Gerard Corbett, chair and CEO of the Public Relations Society of America (PRSA)