I'm often amazed at the stupidity of cows. I encounter these mooing morons frequently on trail runs that pass through pasture lands. Frequently there will be a cow standing astride a path that I'm running along at constant speed, gazing in dumb fascination at me as I approach: 200 meters, 100 meters, 50 meters. Even though I've now been approaching the cow at constant speed in a straight line in the open for the last two minutes, it's often not until I'm within 10 meters that the cow will suddenly realize I'll eventually reach its position, and that's when it suddenly turns in a panic and runs away.
Of course, in no technical sense was my continuing along the same path clearly "predestined", even if you don't believe in free will - any number of plausible forces could intervene to stop me - I could decide to stop because I just ran up a steep hill, or a meteor could hit me, or I could turn left and run into the grass for no reason - but the cows' failure to move until I'm almost on top of them certainly does not result from any nitpicking of causality. Possibly the cows are conditioned by the ranch hands they see more often, and which do stop before they get too close. Or, funnier and just as likely, they're just too damn dumb to recognize the pattern (my straight-line path) and extrapolate it to realize that I'm going to get to their position - not until I'm barely ten meters away.
For the sake of argument let's explicitly assume that there is something in the universe that is not predestined to happen in a certain way at a certain time - and if you'll accept any example, you'll accept that the decay of a single radioactive nucleus fits the bill as non-predetermined. Sure, trillions of them decaying together will fit a pretty nice predictable curve, but it's hopeless to try to predict the decay of individual nuclei; nuclei of the same isotope are, by any measure we know, absolutely indistinguishable, and they decay randomly (that is, without any pattern that we can recognize). The question, then, is: is an individual decay really non-predetermined? Or are we cows that just can't recognize the pattern that they're following? Most importantly, is it possible in principle for a non-pattern-recognizer to ever tell the difference between these two alternatives?
Because the universe contains a finite number of elements, infinite computing power isn't possible; therefore there will never be an infinitely powerful pattern recognizer. Consequently, the question asked in the previous paragraph becomes very important. If it is not possible for a non-pattern-recognizer to tell the difference between inability to recognize a pattern in a system, and actual non-predetermination in that system, then we can never tell if we have free will, or are merely stupid.