My understanding is that the log chain starts at the last full DB backup. So if someone runs a full backup of the DB in the middle of your backup cycle, then deletes that full backup, your log chain is broken.
Hi Jasona,
If someone deleted the full backup, you could still go back to the backup before that, and then restore your chain of log files, since each log backup contains all the records generated since the last log backup. However, if you take differential backups, then any that rely on the deleted base full backup will be unusable.
Copy only backups allow you to take a full backup outside of your scheduled backup routine, and without disrupting your diff backups. Gail Shaw has a nice article on this: http://sqlinthewild.co.za/index.php/2011/03/08/full-backups-the-log-chain-and-the-copy_only-option/
Cheers,
Tony.