During an active stage of development you can always run into one of 3 situations:
- at the moment when you are updating the mirror is syncing with the tier1 mirror and has not yet loaded all packages
- at the moment when a mirror is syncing a packager is just uploading a whole group of packages and the syncing mirror does not get all of the packages until next sync
- a packager is in the middle of his work and only uploaded a part of an update (which is rather unusual), we tend to forget that packagers are human (are they?)
What do you do in this situation? Best advice: wait a few hours or one day and try again.