Reports of bad headers using TUN?

[Matthew]

Hey gang,
  We are running a 3 machine setup using TUN. While we have thousands of 
people using our CGI's on a daily basis, I have this 1 dude (there's 
always frackin' 1) who is getting an internal server error when 
accessing this CGI.

  The apache logs say this when he submits a POST in one particular 
section of the CGI:

request failed: error reading the headers

If he does this same request by directly accessing the nodes, it works fine.

He says nothing has changed in his setup which is T1 -> T1 router -> 
Firewall -> home network.

Is there anything anyone can think of that might be causing this header 
error?  Could there be something in his setup that isn't liking the TUN 
effect? I stray away from this as the reason because other sections of 
the CGI (as well as our whole website) work fine for him.

I can log in as this customer and make the same change and it works fine 
so I'm guessing it has to be something on his side.

Any ideas? Is there some tool I can use or log to read to fix this?

Thanks,
Matthew