Most organizations are using Java 1.7 for their Atlassian applications. If you are still running Java 1.6, then read on!
Java 1.6 is NOT supported on many Atlassian application versions released over the last 2 years! However, some of those versions seem to work fine anyway and administrators may not be aware they needed to upgrade Java when they upgraded Confluence or JIRA for instance. Make sure you are compliant to prevent application problems, problems getting support, or problems with add-ons that require Java 1.7 or above. Moreover, Java 1.6 is no longer being maintained by Oracle for security or other fixes.
Atlassian Java Compatibility
Atlassian documents Java compatibility in their various Supported Platforms pages. Google Supported Platforms for more details for specific applications.
Bob Swift Atlassian Add-ons
Symptoms
The following are symptoms of using the wrong Java version
java.lang.UnsupportedClassVersionError: Unsupported major.minor version 51.0