Aluminium hardens when stretched beyond it's elastic limit, so some aircraft are designed to stretch the material to the plastic region to take full advantage of it.
trouble is, is doesn't go back to it's original shape after the stretching past the elastic limit!! No structural problem really, maybe aerodynamically though.
apparently super guppy's get very buckled
September 08, 2005
Seen taking off. Notice the wrinkles on the forward fuselage. [Canon EOS300D]