When doing retraction with the Ultimaker the quick connect on the extruder allows quite a bit of play.

This shim fits on the quick connect and essentially eliminates this play.

See posts such as this one: https://groups.google.com/d/msg/ultimaker/fNgHD9W6bkI/D5F2PjhWSI8J

This thing was created using the Object Oriented Mechanics Library (OOML). See http://iearobotics.com/oomlwiki/doku.php?id=start for details.