Index: /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 8267)
+++ /trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 8268)
@@ -65,4 +65,6 @@
         @Override
         public void insertString(int offs, String str, AttributeSet a) throws BadLocationException {
+            // TODO get rid of code duplication w.r.t. AutoCompletingTextField.AutoCompletionDocument.insertString
+
             if (selecting || (offs == 0 && str.equals(getText(0, getLength()))))
                 return;
@@ -82,4 +84,12 @@
             if (a != null && a.isDefined(StyleConstants.ComposedTextAttribute))
                 return;
+
+            // if the current offset isn't at the end of the document we don't autocomplete.
+            // If a highlighted autocompleted suffix was present and we get here Swing has
+            // already removed it from the document. getLength() therefore doesn't include the
+            // autocompleted suffix.
+            if (offs + str.length() < getLength()) {
+                return;
+            }
 
             int size = getLength();