Strange send_file problem

Or perhaps I am stumbling in newbie fashion (tho I have active_merchant
and a store site basically up and running in development mode - still I
don't feel I always understand what is going on.)

So, when I have
  send_file(filename, :type => 'application/x-zip')
  redirect :action => index

the send_file seems to hang, rather than complete. I'm deducing this
because of errors that happen after the send_file. When I try to
navigate from the page containing 'send_file' to another page, I get an
error with a trace indicating that it has not been able to complete the
redirect.

If the send_file is followed by a 'redirect :action => index'
the redirect is never executed.

If I comment out the send_file, the redirect is always executed.

What is the problem here? I'm stumped.
thanks much!

Here is the relevant portion of the trace, see the download/redirect
stuff:
er.rb:173:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:95:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:92:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:23:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:82:in `start'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.6/lib/webrick_server.rb:67:in
`dispatch'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.6/lib/commands/servers/webrick.rb:59
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.3.1/lib/active_support/dependencies.rb:147:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.6/lib/commands/server.rb:30
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.3.1/lib/active_support/dependencies.rb:147:in
`require'
script/server:3</code></pre>
    </div>
      <div id="Full-Trace" style="display: none;">

<pre><code>/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.5/lib/action_controller/base.rb:840:in
`redirect_to'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.5/lib/action_controller/base.rb:854:in
`redirect_to'
app/controllers/products_controller.rb:167:in `download_vbj_product'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.5/lib/action_controller/base.rb:941:in
`perform_action_without_filters'

Getting rid of the redirect fixed it, thanks!
note I couldn't use render :action => index either, same problem there.

this group is a great help.