java.lang.LinkageError in Tomcat 6

Usually the cause of this error is you may unintentionally include following two jars into your war file. 
    el-api.jar jasper-el.jar
They are provided by tomcat 6.

In Tomcat 6, you will get following error:

java.lang.LinkageError: loader constraint violation: loader …

You can solve the problem by adding following snippet into your pom file.

<dependency>
        <groupId>org.apache.tomcat</groupId>
        <artifactId>el-api</artifactId>
        <version>[1,)</version>
        <scope>provided</scope>
</dependency>
<dependency>
        <groupId>org.apache.tomcat</groupId>
        <artifactId>jasper-el</artifactId>
        <version>[1,)</version>
        <scope>provided</scope>
</dependency>

Read this post for more information.