Iptables settings can interfere with apt-get update

I ran into a problem on my Ubuntu 12.04.3 box today. I issued the command sudo apt-get update. This usually works without a hitch, but today, it obtained a few of the updates successfully but gave 404 errors for many others. Here's an example:

Err http://security.ubuntu.com precise-security/main Sources
  404  Not Found [IP: 91.189.92.200 80]


After a little thought, I realized how this Ubuntu box differed from others I had worked on: I had used iptables to facilitate running Apache Tomcat on ports 80 and 443. (Because these port numbers are below 1024, Tomcat would have to run as a privileged user to access them, and iptables provides a well-known workaround.)

The particular iptables rule that caused the problem was this one:

sudo iptables -t nat -I OUTPUT -p tcp --dport 80 -j REDIRECT --to-ports 8080


By temporarily deleting that rule (which can be done by replacing -I with -D in the above command), I got sudo apt-get update to work.