What happens when the postamble (0x7F) is not correct?
Since all the CRC checks have been performed already, INIT will not go low. And since the configuration is not yet complete, DONE will not go high. The device will wait in an unknown state. PROG should be toggled to reset configuration. All bitstreams contain the postamble, so this problem is extremely rare. The data would have to be incorrect in exactly the following way: the 0x7F is replaced with 0xFF. If the device never sees a zero, the device will never go past the postamble, and nothing will happen.
If the postamble is incorrect, but does contain a zero (i.e., anything but 0xFF), the device should continue on. However, this might require more CCLKs to bring the device through the startup sequence.