Dave Mason discusses broken log chains:
There are a handful of activities that have broken my SQL Server backup routines. Sometimes a DIFFERENTIAL (or LOG) backup would fail because there was no FULL backup. Other times, a LOG backup would fail because the log chain was broken. Some of those activities include:
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Creating a new database.
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Changing the recovery model to SIMPLE.
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Restoring/reverting a database from a snapshot.
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Any other activity that breaks the transaction log chain.
Dave discusses an easy way of figuring out if you’ve just performed an activity which breaks the log chain.