What will the source of error be?
If it there really is usage of NaCl, I suppose you could say, pH was not maintained. Also, if there's no temperature control, you could mention that, as well.
Parallex error when reading the volume of CO2 produced if a syringe is, in fact, used to take the measurment and there's no counting of bubbles. If you do have to count bubbles, you could say there's a chance that an error could've arisen there.