As you exit the stretching room in the Haunted Mansion you will enter the room with the changing paintings.  Check out the wallpaper in this room.  You will notice that the pattern is in the shape of a classic hidden mickey.

As you enter the Haunted Mansion you will go into a waiting area before you enter the room that stretches.  Along the wall you will notice candlesticks.  If you go under the candlesticks and look up you will see a classic hidden mickey formed by the bottom of the candlesticks.

I will leave this up to you to decide if this is a true hidden mickey.  This one is a bit questionable to me, however it is in the Disneyland’s Hidden Mickey book.