Here is a good example approximately in 00:10 sec
"Hidden" in a sense that the ad doesnot YELL is an advertising but you immediately understand its an advertisement. Here the ad works very well as it has moment of surprise to. Advertiser is called Bradesco who sells car insurance - using hiden ad effect very well.
But is this too annoying at the end? Probably yes if ad is not executed well. But today when tablet ad space is not too growded marketers should run iPad ads like above.