If you see this error while running the code from Eclipse, check the .classpath file for this line:
<classpathentry exported="true" kind="con" path="GROOVY_SUPPORT"/>
In this case, Eclipse adds version 2.1.8 to the classpath and Maven adds 2.1.9. At runtime, Groovy detects both versions and aborts since this will cause subtle bugs.
Solution: Delete the line above in the .classpath file.
If you see a line which contains GROOVY_DSL_SUPPORT, then you can leave that in – the Groovy plugin will use the JARs which Maven adds to the classpath.