Comment 4 for bug 412413

I have just replicated this bug successfully on intrepid:

$ uname -m
i686
$ apt-cache policy zsync
zsync:
  Installed: 0.5-1ubuntu3.8.10.1
  Candidate: 0.5-1ubuntu3.8.10.1
  Version table:
 *** 0.5-1ubuntu3.8.10.1 0
        500 http://at.archive.ubuntu.com intrepid-proposed/universe Packages
        100 /var/lib/dpkg/status
     0.5-1ubuntu3 0
        500 http://at.archive.ubuntu.com intrepid/universe Packages
$ zsync -i karmic-desktop-amd64.iso http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso.zsync
#################### 100.0% 771.4 kBps DONE

reading seed file karmic-desktop-amd64.iso: *************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************Read karmic-desktop-amd64.iso. Target -26.2% complete. ********************************************************************************************************************
downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso:
-------------------- -51.5% 594.6 kBps ETA

downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso:
-------------------- -77.8% 548.7 kBps -115:-57 ETA

zsync works by writing an output file of final size at the beginning, and it does not seem to plan to get the whole DVD here:
$ls -n
total 2542652
-rwx------ 1 1001 1001 729829376 2009-10-15 10:30 karmic-desktop-amd64.iso
-rw------- 1 1001 1001 2083430400 2009-10-16 08:50 karmic-dvd-amd64.iso.part

it aborts:
$zsync -i karmic-desktop-amd64.iso http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso.zsync
#################### 100.0% 771.4 kBps DONE

reading seed file karmic-desktop-amd64.iso: *************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************Read karmic-desktop-amd64.iso. Target -26.2% complete. ********************************************************************************************************************
downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso:
-------------------- -51.5% 594.6 kBps ETA

downloading from http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso:
#################### 100.0% 683.3 kBps DONE 0 ETA

Aborting, download available in karmic-dvd-amd64.iso.part
verifying download...

zsync is no longer running. Never telling the result of "verifying download..."
let's see what we have now:
$ls -n
total 2812628
-rwx------ 1 1001 1001 729829376 2009-10-15 10:30 karmic-desktop-amd64.iso
-rw------- 1 1001 1001 2147483647 2009-10-16 08:54 karmic-dvd-amd64.iso.part

that is longer than the initial output file and the exact length described in
bug 412413.
Just to make sure the file should be longer:
$ wget --spider http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso
Spider mode enabled. Check if remote file exists.
--2009-10-16 09:47:47-- http://cdimage.ubuntu.com/dvd/current/karmic-dvd-amd64.iso
Resolving cdimage.ubuntu.com... 91.189.88.39
Connecting to cdimage.ubuntu.com|91.189.88.39|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 4247142400 (4.0G) [application/x-iso9660-image]
Remote file exists.