Index: trunk/test/unit/org/openstreetmap/josm/data/osm/DataSetMergerTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/data/osm/DataSetMergerTest.java	(revision 9958)
+++ trunk/test/unit/org/openstreetmap/josm/data/osm/DataSetMergerTest.java	(revision 9961)
@@ -993,3 +993,44 @@
         assertEquals(2, w.getNode(1).getId());
     }
+
+    /**
+     * Non-regression test for <a href="https://josm.openstreetmap.de/ticket/12599">Bug #12599</a>.
+     */
+    @Test
+    public void testTicket12599() {
+        // Server node: no modifications
+        Node n1 = new Node(1, 1);
+        n1.setCoor(LatLon.ZERO);
+        assertFalse(n1.isModified());
+        their.addPrimitive(n1);
+
+        // Local node: one modification: addition of an uninteresting tag
+        Node n1b = new Node(n1);
+        n1b.setModified(true);
+        n1b.put("note", "something");
+        assertTrue(n1b.isModified());
+        assertEquals(0, n1b.getInterestingTags().size());
+        my.addPrimitive(n1b);
+
+        // Merge
+        DataSetMerger visitor = new DataSetMerger(my, their);
+        visitor.merge();
+
+        // Check that modification is still here
+        Node n = (Node) my.getPrimitiveById(1, OsmPrimitiveType.NODE);
+        assertNotNull(n);
+        assertEquals("something", n.get("note"));
+        assertTrue(n.isModified());
+
+        // Merge again
+        visitor = new DataSetMerger(my, their);
+        visitor.merge();
+
+        // Check that modification is still here
+        n = (Node) my.getPrimitiveById(1, OsmPrimitiveType.NODE);
+        assertNotNull(n);
+        assertEquals("something", n.get("note"));
+        assertTrue(n.isModified());
+    }
+
 }