Bug 321755 EPS adds a part without considering its placeholder if no target container or part stacks could be found
The summary probably makes no sense. Will deliver a fix with a test in a bit.
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
MPartStack stack = BasicFactoryImpl.eINSTANCE.createPartStack();
		stack.getTags().add("categoryId");
		window.getChildren().add(stack);
		window.setSelectedElement(stack);
		MPartDescriptor descriptor = org.eclipse.e4.ui.model.application.descriptor.basic.impl.BasicFactoryImpl.eINSTANCE
				.createPartDescriptor();
		descriptor.setAllowMultiple(true);
		descriptor.setElementId("partId");
		descriptor.setCategory("categoryId");
		application.getDescriptors().add(descriptor);
		initialize(applicationContext, application);
		getEngine().createGui(window);
		EPartService partService = (EPartService) window.getContext().get(
				EPartService.class.getName());
		MPart createdPart = partService.createPart("partId");
		MPart shownPart = partService.showPart(createdPart, PartState.ACTIVATE);
		assertNotNull(shownPart);
		assertEquals(createdPart, shownPart);
		MPart createdPart2 = partService.createPart("partId");
		MPart shownPart2 = partService.showPart(createdPart2,
				PartState.ACTIVATE);
		assertFalse(shownPart.equals(shownPart2));
		assertTrue(stack.getChildren().contains(shownPart));
		assertTrue(stack.getChildren().contains(shownPart2));
	}
	public void testShowPart_Part_ExistingInNonstandardCategory() {
		MApplication application = ApplicationFactoryImpl.eINSTANCE
				.createApplication();
		MWindow window = BasicFactoryImpl.eINSTANCE.createWindow();
		application.getChildren().add(window);