Index: trunk/src/org/openstreetmap/josm/actions/PurgeAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/PurgeAction.java	(revision 8980)
+++ trunk/src/org/openstreetmap/josm/actions/PurgeAction.java	(revision 8981)
@@ -181,6 +181,6 @@
              */
             List<Relation> relLst = new ArrayList<>(relSet);
-            for (Relation r : relLst) {
-                for (OsmPrimitive parent : r.getReferrers()) {
+            for (int i = 0; i < relLst.size(); ++i) { // foreach loop not applicable since list gets extended while looping over it
+                for (OsmPrimitive parent : relLst.get(i).getReferrers()) {
                     if (!(toPurgeChecked.contains(parent))
                             && hasOnlyIncompleteMembers((Relation) parent, toPurgeChecked, relLst)) {