Why did the gamemakers put mockingjays in the first arena?

It’s stated over and over throughout the series how mockingjays are like a slap to the face of the Capitol. So why would the gamemakers let them inhabit the arena in the first book when it would only lead to more people knowing about them? And it obviously blew up in their faces even more when Katniss made such use of them. Seems like a strange slip on the gamemakers part.