Things you'd expect to be the case, but aren't.
1. People will act in their best self-interest
2. Others will do their part of the project
3. "Your side" is totally right, and the "Other" side is totally wrong
4. People's priorities are the same as yours
5. The deer (or other animal) won't cross the road in front of you.
Murphy's Law corollary - When driving up to an animal, it will take the worse possible action at the worst possible time.
6. The spreadsheet is correct
Check numbers, check formulas, especially if you didn't create it.
7. What's wrong with the car / computer / etc
Check the basics first... have gas? Battery good?... before assuming the worst.
8. There's only one solution to the problem
And your solution is the best.