After upgrading to a new version we sometimes kick off scheduled tasks as a post upgrade step. Examples of these are rebuilding search indexes, or rebuilding the browse_node table in the database.
If a database backup task runs while these tasks are running they will be cancelled, and the upgrade will be incomplete. This is a poor user experience.
Suggestion... perhaps we should have the backup task wait for tasks that are system scheduled to complete before it runs? Or maybe we can somehow flag these tasks as post-upgrade steps, indicating that the backup task should not interrupt them?