My favourite "ambassador" slip was when during a Skyward Sword hands-on event one of the demo guys just up and said there's a "hero mode" or whatever with harder difficulty, fewer hearts, etc, all on camera to the guy playing the game. Just chatting casually about how great it'll be. Yet at the time the mode hadn't been formally announced or confirmed by Nintendo. From memory it took a good few weeks before it was.
My favourite official slip was when the CEO of Nintendo France confirmed in an interview in July 2008 that a new Zelda and a new 3D Mario were in development for the Wii. SMG2 wasn't announced till E3 2009 and Skyward Sword until E3 2010. The guy isn't CEO there anymore lol.