Watch out below is not something you want to have to hear, ever.
A common courtesy when having a roof replacement done to your home is notify neighbors about the upcoming procedure. Debris may not only fall directly down from your roof when removing the old and deteriorated material, but is also may be carried by the wind and can potentially cause damage to something (or someone) on someone else's lawn. Be sure to notify your neighbors and recommend that they move anything that may be in the line of fire of falling roof materials.
On the other hand, a new roof installation is usually a different story. Roof replacement usually involves removal of some or all roofing material and a roof installation typically involves just the install of a new roof on top of bare trusses. This is a much less dangerous situation.