There are a lot of Oracle errors 
out there. Way too many, right?

Quick fix

To a database or other service a connection was requested using a connect identifier. The specified connection identifier could not be resolved to a connection descriptor by using one of the naming methods that are configured. As an example, if the type of connection identifier used was a network service name, the network service name could not be found in a naming method repository, or the repository could not be found or reached.

If you are using local naming (TNSNAMES.ORA file):

• Prove that "TNSNAMES" is listed as one of the values of the NAMES.DIRECTORY_PATH parameter in the Oracle Net profile (SQLNET.ORA)
• Double-check that the TNSNAMES.ORA file exists and is also accessible and in the proper directory.
• Test that the net service name used as the connect identifier exists in the TNSNAMES.ORA file.
• Prove that there are no syntax errors anywhere in the TNSNAMES.ORA file. Look for unmatched parenntheses or stray characters
• It might make the TNSNAMES.ORA files unusable, if there are errors.

If you are using directory naming:

• Double-check that "LDAP" is listed as one of the values of the NAMES.DIRETORY_PATH parameter in the Oracle Net profile (SQLNET.ORA).
• Test if the LDAP directory server is up and accessible.
• Make sure the net server name or database name used as the connect identifier is configured in the directory.
• By specifying a fully qualified net service name or a full LDAP DN as the connect identifier you authenticate that the default context being used is correct.

If you are using easy connect naming:

• Find out if "EZCONNECT" is listed as one of the values of the NAMES.DIRETORY_PATH parameter in the Oracle Net profile (SQLNET.ORA).
• Service name, host and port have to be correct.
• The connect identifier should be in quote marks enclosed. For further details on naming check the Oracle Net Services Administrators Guide or the Oracle operating system guide. 

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/net12150.htm#ORA-121504

Quick fix

The listener received a request to set up a connection to a database or other service. The connect descriptor that has been received by the listener specified a service name for a service (usually a database service). Either it has not been dynamically registered with the listener yet or has not been configured statically for the listener. This can be a temporary condition, for example, after the listener has been started, but before the database instance has registered with the listener.

Hint:

• Wait a moment and try to reconnect.
• Find out which services are currently known by the listener by executing: lsnrctl services <listener name>
• Dopple-check that the SERVICE_NAME parameter in the connect descriptor of the net service name used specifies a service known to the listener.
• If an easy connect naming connect identifier was used, prove that the service name specified is a service known to the listener.
• Check in the listener.log file for an event.

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/net12500.htm#ORA-12514

Quick fix

A generic protocol adapter error occurred.

Hint:

Authenticate the addresses used for correct protocol specification. Before you report this error, look at the error stack and double-check for lower level transport errors. For more details, turn on tracing and perform the operation again. Turn off tracing when the operation is complete.

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/net12500.htm#ORA-12560

Quick fix

Backtrace message if the stack is processed by unhandled exceptions.

Hint:

Fix the problem that is the reason for the exception. You can also write an exception handler for this condition. You might need to contact DBA or your application administrator.

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/e4100.htm#ORA-06512

Quick fix

The listener received a request to set up a connection to a database or other service. The connect descriptor that has been received by the listener specified a SID for an instance (usually a database instance). Either it has not been dynamically registered with the listener yet or has not been configured statically for the listener. This can be a temporary condition, for example, after the listener has been started, but before the database instance has registered with the listener.

Hint:

• Please wait a moment and try to reconnect.
• Find out which services are currently known by the listener by executing: lsnrctl services <listener name>
• Dopple-check that the SERVICE_NAME parameter in the connect descriptor of the net service name used specifies a service known to the listener.
• If an easy connect naming connect identifier was used, prove that the service name specified is a service known to the listener.
• Check in the listener.log file for an event

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/net12500.htm#ORA-12500

Quick fix

This is the generic internal error number for Oracle program exceptions. This points to a process encountering an abnormal condition.

Hint:

Report it as a bug – the internal error number is the first argument.

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28278/e0.htm#ORA-00600

Quick fix

The alert log is an XML file which is a chronological log of database messages and errors. It is stored in the ADR and contains messages on the following points:

• Critical errors (incidents)
• Administrative operations, such as starting or shutting down the database, restoring the database, creating or deleting a tablespace, and others.
• Error when automatically updating a materialized view
• Other database events

Hint:

View the alert log in text format (with the XML tags removed) using Enterprise Manager and the ADRCI utility. Also in the ADR there is a text-formatted version of the alarm log stored for backward compatibility. Having said that Oracle recommends that any parsing of the alert log contents can be done by using the XML-formatted version, as the text format is unstructured and can change from release to release.

Best solution:

https://docs.oracle.com/cd/B28359_01/server.111/b28310/diag001.htm#ADMIN11260

More solutions:

https://docs.oracle.com/cd/B28359_01/server.111/b28310/diag005.htm#ADMIN11267