This error is likely to happen when your version of JDK is lower than required by Tomcat. On Mac computers, even if you have chosen Java 6 for Tomcat 7 via the utility Application/Java Preference, the one used by Tomcat is not 6.
To solve this problem, modify Tomcat/bin/startup.sh, add this line after the line "executable=catalina.sh" to use Java 6 (change 1.6 to other versions required).