Version:            (using KDE KDE 3.5.6)
Installed from:    Debian testing/unstable Packages

This has been reported in Debian BTS at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=272668

If I use konqueror to see
http://www.eleves.ens.fr/home/glisse/bugmng/anim_mod_01.mng
I get messages like:
MNG error 999: Internal error in libmng; chunk IHDR; subcode 0:0
MNG error 11: Function is invalid at this point; chunk IHDR; subcode 0:0
(this second line actually appears 8 times)

    I am able to reproduce this bug using konqueror as well as
showimg. I downloaded the anim_mod_01.mng and when I ran it using

$konqueror anim_mod_01.mng

The face in the image changes for sometime and then hangs. The errors
on the konsole are

konqueror: WARNING: Can't open
MNG error 999: Internal error in libmng; chunk IHDR; subcode 0:0
MNG error 11: Function is invalid at this point; chunk IHDR; subcode 0:0

and the last error repeated for 8 times. However if I do

$showimg anim_mod_01.mng

The behavior is same. The errors on the konsole are

MNG error 999: Internal error in libmng; chunk IHDR; subcode 0:0
MNG error 11: Function is invalid at this point; chunk IHDR; subcode 0:0

here the second error repeats for 63 times instead of just 8 times.

The file works fine using animate.