Index: trunk/src/org/openstreetmap/josm/gui/dialogs/relation/MemberTableModel.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/relation/MemberTableModel.java	(revision 7551)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/relation/MemberTableModel.java	(revision 7552)
@@ -467,4 +467,8 @@
             return;
         for (int row : idx) {
+            // fix #7885 - IndexOutOfBoundsException: Index: 39, Size: 39
+            if (row >= members.size()) {
+                continue;
+            }
             RelationMember oldMember = members.get(row);
             RelationMember newMember = new RelationMember(role, oldMember.getMember());