What assumptions guarantee late projects?

For traditionally-managed projects, two assumptions guarantee project completion delays: 

1️⃣ Project task times is predictable, and 

2️⃣ Traditional project management system is effective. 

We ask resources to provide an estimate of the time required to complete a particular task. 

Once resources have reported their estimated (and safe) times, management requires lower estimates. 
If everyone accepts these estimates, the estimate becomes a commitment. Resource is then evaluated on their commitment.