The following code throws a "ClassFormatException: Invalid method signature: TT;" when executed:
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import org.apache.bcel.Repository;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.MethodGen;

class MyClassLoader extends ClassLoader {
	@Override
	public Class<?> loadClass(String name) throws ClassNotFoundException {
		if (name.startsWith("java.lang"))
			return super.loadClass(name);

		JavaClass jcl;
		jcl = Repository.lookupClass(name);
		scanMethods(jcl);
		byte[] data = jcl.getBytes();
		return defineClass(name, data, 0, data.length);
	}

	private void scanMethods(JavaClass jcl) {
		ConstantPoolGen cpg = new ConstantPoolGen(jcl.getConstantPool());
		Method[] methods = jcl.getMethods();
		for (int i = 0; i < methods.length; i++) {
			MethodGen mg = new MethodGen(methods[i], jcl.getClassName(), cpg);
		}
	}
}

class Foo<T> {
	public void bar(T x) {
	}
}

public class MethSignTest {
	public MethSignTest() {
		Foo<Integer> foo = new Foo<Integer>();
	}

	@SuppressWarnings("unchecked")
	public static void main(String[] args) {
		try {
			MyClassLoader mcl = new MyClassLoader();
			Class<MethSignTest> c = (Class<MethSignTest>) mcl
					.loadClass(MethSignTest.class.getName());
			c.newInstance();
		} catch (Exception e) {
			e.printStackTrace();
		}

	}
}
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Here's the full stack trace:

org.apache.bcel.classfile.ClassFormatException: Invalid method signature: TT;
	at org.apache.bcel.classfile.Utility.typeOfSignature(Utility.java:986)
	at org.apache.bcel.generic.Type.getType(Type.java:169)
	at org.apache.bcel.generic.MethodGen.<init>(MethodGen.java:255)
	at methodsignature.MyClassLoader.scanMethods(MethSignTest.java:26)
	at methodsignature.MyClassLoader.loadClass(MethSignTest.java:17)
	at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
	at methodsignature.MethSignTest.<init>(MethSignTest.java:38)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:501)
	at java.lang.Class.newInstance0(Class.java:350)
	at java.lang.Class.newInstance(Class.java:303)
	at methodsignature.MethSignTest.main(MethSignTest.java:47)

This misbehaviour occurs at least since revision 617860.