While Google (NASDAQ:GOOGL) has seemed nearly unstoppable lately, that is exactly what investors had once thought about Microsoft and, more recently, Apple. In the video below, Fool.com contributor Doug Ehrman discusses three potential issues that the search thing could face which will be important for investors to monitor. First is the fact that Google's performance had made the stock fairly expensive on a valuation basis. Second, while Google expects to see significant growth in mobile advertising, the paradigm shift in online ads could shake things up. And finally, as we all learned with Apple, a company's image as an innovator can change quickly in a short amount time.

The three concerns discussed are not necessarily harbingers of doom, but rather things to watch for as time goes by and Google looks to maintain dominance.

Fool contributor Doug Ehrman has no position in any stocks mentioned. The Motley Fool recommends Apple and Google. The Motley Fool owns shares of Apple, Google, and Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.