That pack with the 48 "essentials", 5 days of food, 6 knives, 4 flashlights, etc. will weigh a lot more that it did when you packed it.
The clear blue skies will disappear and it will pour down rain.
That hotspot will blister by the time you check it even though you "don't get blisters".
You'll hit a confusing junction with a cross-trail.
You will twist an ankle.
Something will come lose: a pack strap, a boot sole, etc.
It will turn out to be some bug's annual activity peak.
You'll have to poop when you didn't plan to.
Someone on an ATV or mountain bike will almost hit you.
The trail will be flooded.
You'll pick up a tick and not notice it until you're scratching yourself in bed that night.
Someone will break into your car at the trailhead.

Every one of these has happened to me at least once except the car break-in.