While looking over my SQL Server Error logs, I noticed a small flurry of error messages like you see below, every two hours, at the top of the hour:
Login failed for user 'PRODUCTION\SomeSQLAccount'. Reason: Failed to open the explicitly specified database. [CLIENT: 192.168.xxx.xxx]
Error: 18456, Severity: 14, State: 38.
The fact that it happened at exactly the same time, every two hours, led me to believe that the error messages were being caused by a SQL Agent job. When I looked closer at my SQL Agent jobs, I quickly discovered the culprit, which was a job that was trying to access a database that was mirrored to another server. That was a DBA error that was easy to fix.
If you are actually having login failed errors, the resources below can help you figure out what is actually going on.