When comets are set in "Data Shown.." on "Auto", decrease the stars
magnitude to the point where a comet is about to disappear, its position
changes about a magnitude before the event. After the disappearance,
increasing the magnitude reverses the process, when the comet reappears
it will move when magnitude is further increase by one.

If one creates a track as the comet appears and then decreases the field
which increases magnitude the comet changes position and the track
become invalid.

We tried on 3 different computer systems and replicated the error with
more than one comets. Guide version is from 17 June 2013.

This only append when a comet is about to disappear or reappear.

Hope this is clear enough explanation to find a fix.

Denis