Facebook hasn’t even gone public yet and it looks like the company’s IPO honeymoon may be over before it starts.

Today, a consumer internet behemoth with which Facebook has maintained a relatively friendly relationship with, Yahoo, slapped the social networking giant with a lawsuit alleging that it is infringing on 10 Yahoo patents which focus primarily on advertising, but also include, for instance, an “Online playback system with community bias.”

Reports that Yahoo and Facebook were trying to resolve patent-related matters surfaced recently, and the Palo Alto-based social network updated the risks section of its S-1 filing to list patent-related lawsuits.

That was an appropriate update, as the Yahoo lawsuit comes little more than a few weeks after the companies’ discussions came to light. “Unfortunately, the matter with Facebook remains unresolved and we are compelled to seek redress in federal court,” Yahoo’s statement reads.

While it’s still possible the parties will settle, the news isn’t good for Facebook. As Reuters notes, “most [patent lawsuits in the social networking space] have been filed by patent aggregators that buy up intellectual property to squeeze value from it via licensing deals and none by a large tech company such as Yahoo.”

That means that Yahoo, which once reportedly tried to buy Facebook, could prove to be a very different adversary than Facebook is used to — one which is just as interested in defending its turf on the consumer internet as it is in finding a pot of gold at the end of the rainbow.