Linuxluver:
If you stack them warm / hot, they contract as they cool. So does any gas trapped in the vessel below by the one above.
It's most likely it isn't the glasses that aren't stacking properly so much as the person stacking the glasses isn't doing it properly by failing to allow them to cool down. :)
Proper ones have a step in the side of them that is where they stop so that doesnt happen.
