The unenforceable Continental System.
The Spanish invasion that tied down an Army in Spain to fight irregulars.
The lack of a first rate Navy that could compete with the Royal Navy.
The loss of the Slave Revolt in Haiti.
Not consulting with a weatherman before invading Russia.
Reliance on muskets instead of the slower firing but morer accurate and longer range rifle.
Attacking at Waterloo with a poorly trained Army.