Ok, this is taken from the C3C manual:
"Preserve Random Seed: When this option is checked, the state of the current game's random number generator is locked when you save a game, so that saving and loading have no effect on random events in the game. For example, right after you save your game, one of your units is attacked and destroyed by an enemy unit. You can reload your last save and allow the combat to occur again. If the Preserve Random Seed is not checked, a new random number is generated when the game is reloaded, and the results of the combat might be different. If the Preserve Random Seed option is checked, the result of the combat is always the same because the original random number that governed the combat is preserved in each save."
This applies to spy missions too...
Remember that this is a way to CHEAT!