Weird tr bug?

Is this a bug or am i clueless?  The following happens on every Linux box
i have tried it on (bash 1.x or bash 2.x):

biteme:~$ mkdir temp
biteme:~$ cd temp
biteme:~/temp$ echo XoX | tr [A-Z] [a-z]
xox
biteme:~/temp$ touch m
biteme:~/temp$ echo XoX | tr [A-Z] [a-z]
mom
biteme:~/temp$ mv m p
biteme:~/temp$ echo XoX | tr [A-Z] [a-z]
pop
biteme:~/temp$ rm p
biteme:~/temp$ echo XoX | tr [A-Z] [a-z]
xox

This also works on an old BSDI box i have (BSDI vers 2.01 using /bin/bash
or /bin/sh).  If this is a bug, it must be a very old one.  The man page
for tr on this box is dated 1993.

Regards,
Bill