Always operate as if things will fail. Therefore you will either congratulate yourself on your steely-eyed prescience, or be very pleasantly surprised.
Motto.
No lie, this is possibly the #1 thing I have learned about managing projects. Well. Maybe #2, after “COMMUNICATECOMMUNICATECOMMUNICATE.” If you don’t feel like you’re spending at least 75% of your time telling everybody else what is going on, or else chasing down people to find out what is going on, then you’re doing it wrong.
But you should feel like you’re spending the other 25% making contingency plans and alternative budgets.