Good Film Condition

Make sure the film is in good shape!  There is no point in trying to run a torn and poorly spliced film through any projector.  Especially make sure that the beginning of the film is clean and free of any physical defects.
Bent, torn, rough, jagged (etc.) edges will definitely cause problems when threading the film.
Torn sprockets can cause the projector to loose its upper or lower loops which then removes all the slack in the film and can lead to further sprocket damage, or breaking the film.
Many old home movies that have splices will tend to be brittle and dry.  If the splice was an old edge (butt) splice, they can break easily.  Tape splices will usually hold together.