You must have applied the save to "All events in the series" for the first lesson after you put the notes in. It is a part of the description of the event so that is why it keeps showing up. To fix this, go to the first lesson these notes were in, edit the lesson, and delete the information in the description field. Then Save and apply the changes to "All events in the series. You will need to do this for each lesson that has this problem.