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:
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.