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[Glider]

Yeah they do that no matter what.

Like amazonian said about the sunken backs, I have personally had that happen to very young juveniles who constantly stood up in the corners. Only thing I can suggest to help prevent that is to try and build up the corners so they cant sit at right-angles. I used to make little climbing gyms (like at mcdonalds!) out of toilet rolls and shoeboxes and stuff to keep them occupied as much as possible. Lots of shredded newspaper helps too.