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Although, I'll be honest I don't recognise the error message you're getting I can see a bug in your code.

self.image_sub = rospy.Subscriber("/kinect/rgb",Image,self.callback_rgb)      #Case 1
self.image_sub = rospy.Subscriber("/kinect/depth",Image,self.callback_depth)  #Case 2

In these two lines you're writing the subscriber objects to the same variable self.image_sub so the second one should destroy the first. Try changing the these to two different identifiers, and see if the error continues.