By Chris Ciaccia

Marissa Mayer was named Yahoo (YHOO) CEO on July 16, and since then she has enjoyed a cozy relationship with the media and Wall Street as she tries to turn around the struggling Internet giant. The gloves came off yesterday, though, as one of Yahoo's top properties, its fantasy football site, was down on a crucial day.

As Mayer looks to turn Yahoo around and compete with the likes of Google (GOOG) (her former employer), Facebook (FB), and other companies supported by advertising revenue, she is increasingly looking to focus on mobile, and the company's apps, which are lacking. Yahoo even made its first acquisition under Mayer recently, grabbing the iPhone app Stamped.

On the conference call discussing third-quarter earnings, Mayer noted that Yahoo has not done a great job with its mobile efforts. "And while we've made progress, Yahoo hasn't capitalized on the mobile opportunity. We haven't effectively optimized our websites, we've underinvested in our mobile front-end development and we've splintered our brands," she said. "We have more than 76 applications across Android and iOS. All of this needs to change. Our top priority is a focused, coherent, mobile strategy. We're accelerating our efforts to build a strong technical talent base for mobile. This includes engineers, product managers and designers."

If focusing on mobile apps and building a better experience for your users involves one of your most important properties suffering a severe outage, then a significant amount of work needs to be done. It's certainly not a great way to start.

The fantasy football property first went down around 12 p.m. EST on Sunday, and was down most of the day, according to several users. This is more than just an annoyance to fantasy sports players such as myself, as fantasy sports has become big business. Not only for the players themselves in terms of prize money, but for Yahoo! too, which gets significant amounts of traffic.

Mayer's Twitter account was flooded with angry users who could not access the app. Here are just a few of the mentions she received, including one from myself:

 The Sunnyvale, Calif., company sent out an email very late Sunday night apologizing to users, trying to stem the issue, but that won't help the millions of users that lost access, and subsequently lost their match-ups because Yahoo's servers went down.

As Yahoo tries to reinvent itself for the 21st century and focus more on smartphone and tablets, having one of your main properties go down at the most crucial time does not build a good rapport between Yahoo and its users.

Yahoo shareholders have enjoyed an 8.42% return over the past month, as Mayers' honeymoon period continues in light of stronger-than-expect​ed third quarter earnings and a good relationship with Wall Street.

More mistakes like this, though, and that relationship will go from smelling like candy to sour grapes very fast.

Interested in more on Yahoo? See TheStreet Ratings' report card for this stock.

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