The original description isn't clear enough to explain the problem. I've contacted the submitter, and we've agreed on the following description:

SETUP REQUIRED:

Go to your home directory and make sure it has the following files:
- "a"
- "b.gif"
- no other GIF files
On Windows XP this is your "My Documents" directory.

NOTE:
Reproducing this bug requires you to stay in the same directory.
Please follow the steps directly and don't change out of the default
directory.

STEPS TO REPRODUCE:

- Launch the JFileChooser demo
- Select "Show JPG and GIF filters"
- Select "Motif"
- Push "Show FileChooser" button
- Ensure that the "*" filter is chosen.
- Select "a" file from the "Files" list on the right
- Toggle filter menu and select "GIF image files (.gif)"
- Try to select "b.gif" file from the "Files" list on the right.
  (It's already selected)
- Though the item in the list is selected, the filename doesn't
apper in "Enter file name:" text field below.
- As a result, double-clicking "b.gif" closes the dialog and does nothing.
  Hitting OK instead, returns file "a".

Original Description
--------------------
When opening a file using JFileChooser with MotifLook&Feel "File does not exist" error is thrown. This happens when you switch Filter between .x and .y and try to open the highlighted file in the "Files box".