[Mr V]

As Ether noted the flexibility of some plastics can vary greatly depending on temperature. Personally I squashed a ball by stepping on it and thought it would certainly be damaged but I watched it pop out on its own. Granted I did realize that I was stepping on something and didn't put my full weight on it. That took place in a warehouse with the thermostat sat at 60 degrees and the balls had been in their long enough that they should been close to room temp.

Since we have several reports of cracking, yet others report that they have survived serious deformation, I have to wonder what the temp was in the location where they were being testing and stored in each case.