If the Shells were on display in sunlight, the exposed side can warp do to heat & UV.
Old hobby shops with big front windows often had damaged to display items and those in window type boxes.
Faded cardboard boxes are seen at swap meets all the time.
I now always check for warping on used models, I was "burned" recently on a $15 boxed Darpol LMS HO loco with hidden sun damage and warping.
Now I just claim it was in a train wreak and badly repaired by the roundhouse crew.