On starting Confluence, do you see an UnsupportedClassVersionError error that causes Confluence to close, similar to the one below?

Cause: 
some.plugin.class.here: java.lang.UnsupportedClassVersionError: some/plugin/class/here (Unsupported major.minor version 49.0)

This is caused by installing a plugin that requires a newer version of Java than the version you have currently installed. You can either install the latest JDK, or uninstall the plugin:

Install The Latest JDK

1. Visit the Java Downloads page.
2. Locate JDK 5.0 Update 8, the option without NetBeans or Java EE, and select 'Download'
3. Check the 'Accept' box for the License Agreement
4. Under the Windows Platform, select the 'Windows Offline Installation, Multi-language' version
5. Begin downloading the executable
6. Once downloaded, run the installer. You will need to remember the directory you install the JDK to.
7. Windows users follow the instructions to Setting the JAVA_HOME Variable in Windows. This will set your JAVA_HOME environment variable to the directory you just installed the JDK in. The default directory is under C:\Program Files\Java.

Uninstall Problem Plugin

One of the plugins you just installed will be the cause of the problem. Plugins are installed as JAR files, which you can manually remove. Your error message contains a plugin class that indicates which plugin is causing the problem. Known plugins are:

The plugin JAR file will be in one of the following Confluence plugin directories:
Confluence Home/plugins
Confluence Install/confluence/WEB-INF/lib

To locate the plugin JAR and remove it:

1. Visit the plugin page and locate the Download section
2. Find the filename of the JAR file by checking the download URL for the plugin
3. Locate the JAR file in one of the Confluence plugin directories
4. Remove the plugin JAR file
5. Start Confluence