Some guy designs a "I want to have motivation" placebo and lo and behold he decides to try. It's almost as if he wanted it to work...
This is different from fooling yourself about knowledge that you don't have (rather than knowledge that you do) if the only way a caster can gain the option for the save line in the first place is if he doesn't already know his spell is an illusion. There are ways to hide information from yourself, but its a dangerous road to go down.