In no particular order:
1. No matter how much food you eat your pack will not get lighter as the days go by.
2. No matter how much food you eat you will not have any more room in your pack as the days go by.
3. You will only need the one item in your first aid kit that you did not bring with you.
4. You can never bring too much toilet paper.
5. The amount of mosquitoes on your trail will be in inverse proportion to the amount of repellent you have brought with you.
6. You will spend hours finding good bear caches but never see any bears.
7. Distances marked on your map will not correlate in any way to the time taken to hike them.
8. Bring a pack cover and it will not rain. Forget it and you will hike in a downpour for your entire trip.
9. Your meals get more basic but taste better as the days go by.
10. Notwithstanding point 9 the fast food you wolf down when immediately back in civilisation will be the best thing you've ever eaten.
Have we missed any?