Hi:
Actually the error are accessing the elements of the model inside the structure. Ej: entity.model.x, entity.model.tilt
Maybe i'm doing something wrong.....

unction main()
{
entity3d enemy;
enemy.model = ent_create ("mymodel.mdl", vector(0,0, 25), NULL);
func(enemy);
}
function func(entity3d* ent)
{
VECTOR v1;
ent.state = MODE_WALK;

v1.x = ent.model.x; // Error and crash!
vec_set (v1.x, ent.model.x); // Error and crash
ent.model.y -= 1; // Error and crash!
}

NOTE:
The problems disappear when the structures are declared globally. Not inside a function. Still investigating. (reading the manual)
Thanks for your corrections....