Given the following java classes:

public abstract class Simple {
    public Simple getSimple() {
        return this;
    }
}

class NotSoSimple extends Simple {
    @Override public NotSoSimple getSimple() {
        return this;
    }
}

class Complex extends NotSoSimple {
    @Override public Complex getSimple() {
        return this;
    }
}

And the following Groovy Script:

class CovariantProblem {
  static void main(args) {
    	def complex = new Complex()
    	complex.getSimple()
  }
}

Groovy complains that there's no method getSimple() in Simple class.
The problem occurs both in Groovy Eclipse plugin and in Groovy classloader, as shown in the exception above:

Exception in thread "main" java.lang.NoSuchMethodError: Simple.getSimple()LComplex;
at gjdk.Complex_GroovyReflector.invoke(Unknown Source)
at groovy.lang.MetaMethod.invoke(MetaMethod.java:115)
at org.codehaus.groovy.runtime.MetaClassHelper.doMethodInvoke(MetaClassHelper.java:713)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:560)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:450)
at org.codehaus.groovy.runtime.Invoker.invokeMethod(Invoker.java:119)
at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.java:111)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodN(ScriptBytecodeAdapter.java:187)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethod0(ScriptBytecodeAdapter.java:211)
at CovariantProblem.main(CovariantProblem.groovy:6)