java.lang.UnsupportedClassVersionError: Unsupported major.minor version 51.0

One of my programming guys was getting this error:

java.lang.UnsupportedClassVersionError:  Unsupported major.minor version 51.0

and Tomcat failed to start.

The message from the tomcat logs was:

SEVERE: Catalina.start: 
org.apache.catalina.LifecycleException: Failed to start component [StandardServer[8005]]

The fix was simple.  He was compiling at JRE version 1.7 and the JRE on the server was 1.6.  This was easily fixed in Eclipse under Project Properties > Java Compiler.

Hope this helps someone.