Are you choosing when to sync? When did the error in this sync occur? Date and time?
Over the past several weeks there have been intermittent function issues with the Member Tree program.
I recommend to:
1) Turn off auto-sync and leave it off.
2) Before performing a sync, spend some time using functions on the on-line member tree program, if everything appears smooth, go ahead and sync.
3) Check the Member Tree and Ancestry Site Comment boards for recent posts about wonky function.
4) Don't sync on a Friday evening, the airing of *Who do you think you are* burdens the servers with new traffic. The weekend never seems to be a good time to sync in general.