Index: trunk/src/org/openstreetmap/josm/actions/mapmode/ExtrudeAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/mapmode/ExtrudeAction.java	(revision 3938)
+++ trunk/src/org/openstreetmap/josm/actions/mapmode/ExtrudeAction.java	(revision 3939)
@@ -303,5 +303,5 @@
                     //find if the new points overlap existing segments (in case of 90 degree angles)
                     Node prevNode = getPreviousNode(selectedSegment.lowerIndex);
-                    boolean nodeOverlapsSegment = prevNode != null && Geometry.segmentsParralel(initialN1en, prevNode.getEastNorth(), initialN1en, newN1en);
+                    boolean nodeOverlapsSegment = prevNode != null && Geometry.segmentsParallel(initialN1en, prevNode.getEastNorth(), initialN1en, newN1en);
                     boolean hasOtherWays = this.hasNodeOtherWays(selectedSegment.getFirstNode(), selectedSegment.way);
 
@@ -320,5 +320,5 @@
                     //find if the new points overlap existing segments (in case of 90 degree angles)
                     Node nextNode = getNextNode(selectedSegment.lowerIndex + 1);
-                    nodeOverlapsSegment = nextNode != null && Geometry.segmentsParralel(initialN2en, nextNode.getEastNorth(), initialN2en, newN2en);
+                    nodeOverlapsSegment = nextNode != null && Geometry.segmentsParallel(initialN2en, nextNode.getEastNorth(), initialN2en, newN2en);
                     hasOtherWays = hasNodeOtherWays(selectedSegment.getSecondNode(), selectedSegment.way);
 
Index: trunk/src/org/openstreetmap/josm/tools/Geometry.java
===================================================================
--- trunk/src/org/openstreetmap/josm/tools/Geometry.java	(revision 3938)
+++ trunk/src/org/openstreetmap/josm/tools/Geometry.java	(revision 3939)
@@ -290,5 +290,5 @@
     }
 
-    public static boolean segmentsParralel(EastNorth p1, EastNorth p2, EastNorth p3, EastNorth p4) {
+    public static boolean segmentsParallel(EastNorth p1, EastNorth p2, EastNorth p3, EastNorth p4) {
         // Convert line from (point, point) form to ax+by=c
         double a1 = p2.getY() - p1.getY();
@@ -300,5 +300,7 @@
         // Solve the equations
         double det = a1 * b2 - a2 * b1;
-        return Math.abs(det) < 1e-13;
+        // remove influence of of scaling factor
+        det /= Math.sqrt(a1*a1 + b1*b1) * Math.sqrt(a2*a2 + b2*b2);
+        return Math.abs(det) < 1e-3;
     }