Privoxy breaks united.com flight search for me. Try replicate:
1. Go to http://united.com/.
2. Log in with your united.com account. (You will need to have an account on united.com. If you don't already have one, you can probably create a frequent flier account with false information for testing purposes.)
3. Go back to http://united.com/.
4. Hover your mouse over the tab labelled "Planning & booking" (in the upper-left column), and click on "flights".
Actual observed (broken) behavior with Privoxy:
Nothing happens. The browser spinner spins, but the flight search page is never loaded.
The browser loads the flight search page successfully.
I have verified that with Privoxy disabled, I see the desired behavior, but with Privoxy enabled, I see the broken behavior listed above.
Note that it is important to log in with a United.com account first. If you don't log in, everything will work fine. Privoxy only breaks the united.com site when you are logged in.
I am attaching the log entries that are generated in the Privoxy logfile, when performing step 4, with Privoxy configured for full logging. I have redacted all possibly-sensitive cookie values, to preserve my own security and privacy, but I don't think those specific values are likely to be relevant. Notice the final entry: "Connect: Stopping to watch the client socket. There's already another request waiting."
I am using Privoxy on Fedora 14, with the privoxy-3.0.16-3.fc14.x86_64 RPM. The output of uname -a is "Linux senfl.cs.berkeley.edu 184.108.40.206-92.fc14.x86_64 #1 SMP Sat May 21 17:26:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux". I am using Firefox 3.6.23: firefox-3.6.23-1.fc14.x86_64. Note that I have the HTTPS Everywhere extension enabled. I don't know if that is relevant.