Samsung is a sponsor of the Mutua Madrid Open tennis tournament that begins this weekend. Ferrer intended to send out a tweet stating he was very happy with his #GalaxyS4 and that he was "configuring S Health on my new #GalaxyS4 to help with training @SamsungMobile."
Ferrer likely didn't realize that Twitter shows which app a tweet is posted from. In this case, as you can see in the screenshot below, his tweet was posted from Twitter for iPhone.
The tweet was quickly taken down but not before being spotted by Jaime Novoa and the Huffington Post.
This isn't the first time this sort of thing has happened. Oprah's account tweeted to promote the Microsoft Surface from an iPad and Alicia Keys claimed her account was hacked when caught tweeting from an iPhone after being named BlackBerry's Global Creative Director.