I've been downloading disk image files (.dmg) containing Mac utilities and applications with no problem for years, but all of a sudden, a particular update .dmg always downloads in a corrupted form. I've been trying to update Mathematica 7, but the disk image containing the updater consistently gives "codec overrun" errors when it is being verified prior to mounting.

The disk image can be mounted by clicking "skip" during the verification process, but the files are corrupted. They look OK, but the installer errors out a few seconds after launching.

Apparently this .dmg corruption issue has to do with using SPI (Stateful Packet Inspection) in our Netgear router's firewall. I haven't yet had the opportunity to test it without the SPI setting, but according to many people, this will resolve the problem in most cases.

To fix, open your Netgear router's administration page (http://www.routerlogin.net), log in and go to WAN settings. Uncheck SPI.