1.) When the steel balls are falling down, there are many directions that the experimenters can see the line of start and stop. This may cause an error of time.
2.) When you put the steel balls on the digital scale, the air from air-conditioners will cause an error of mass in the experiment.
3.) The steel balls may hit on the glass tube if the experimenters drop them near the edge of glass tube. This may cause an error in the experiment.
4.) There are many heights that the experimenters can drop the steel balls, but the different height may cause an error of time.
5.) The given steel balls are covered with the oil which has a mass that may cause an error of mass in the experiment.