A possible error can happen when you put the rubber stopper back onto the Erlenmeyer flask. If you do not secure the stopper onto the flask there may be room for the Hydrogen gas to escape which would significantly change the results of the experiment. With altered results the final conclusion of the experiment would be inaccurate which would render the experiment obsolete. Another possible source of error is if you allow oxygen to go to the top of the gas collector and forget to record the volume that it takes up. This would make your data a little less accurate and therefore reduce the quality and precision of your experiment.
