I used my CF card in a 20D for a couple of shots. When I put it back in my
G3 the file number of the next image was reset from IMG_5846 to IMG_0801.
Oh no!
So I went onto the CF card (using my card reader) and opened the folder called
DCIM\CANONMSC. In there were two files. The one named 308.CTG I
renamed to 158.CTG, then I opened the file in a text editor (not notepad)
and changed the text at the beginning of the file in the same manner.
For good measure (and to continue pics at the right number), I copied my
last pic IMG_5846.JPG back to the DCIM\158CANON folder on the CF card
and then took a picture.
My file numbers are back to normal now! Whew...that was close.

