# Amazon EMR: Custom JAR cluster errors
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The following errors are common to custom JAR clusters.

**Topics**
+ [Is your JAR throwing an exception before creating a job?](#emr-troubleshoot-error-custom-jar-1)
+ [Is your JAR throwing an error inside a map task?](#emr-troubleshoot-error-custom-jar-2)

## Is your JAR throwing an exception before creating a job?
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 If the main program of your custom JAR throws an exception while creating the Hadoop job, the best place to look is the `syslog` file of the step logs. For more information, see [View Amazon EMR log files](emr-manage-view-web-log-files.md). 

## Is your JAR throwing an error inside a map task?
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 If your custom JAR and mapper throw an exception while processing input data, the best place to look is the `syslog` file of the task attempt logs. For more information, see [View Amazon EMR log files](emr-manage-view-web-log-files.md).