It's not possible to use the relatively popular search engine

   http://www.altavista.com/

with apache2's mod_proxy* modules.

You can easily see the problem, if you

  a) type a search word into the search field in 
     http://www.altavista.com/

  b) click on of the links in this page

The main problem is, that apache-mod_proxy does some URL re-encodings. After
this re-encodings the original URL path component differs from the encoded form.

An example. There is an example link from http://de.altavista.com/ (I
changed it a little bit, because I do not know, if the URL contains
private infos)
  
http://av.rds.yahoo.com/_ylt=A9ibyDZZCEq4AklmSLaMX;_ylu=X3oDBvNjNnZmYzBHBndANhdl93ZWJfaG9tZQRzZWMDdGFicw--/SIG=11nr22kc/EXP=111216420/**http%3a//de.altavista.com/dir/default

apache-mod_proxy transforms it to (sniffed with ethereal):

   GET
/_ylt=A9ibyDZZCEq4AklmSLaMX;_ylu=X3oDBvNjNnZmYzBHBndANhdl93ZWJfaG9tZQRzZWMDdGFicw--/SIG=11nr22kc/EXP=111216420/**http://de.altavista.com/dir/default
   HTTP/1.1

Do you see the difference? "http%3a//" is transformed to "http://". 

The offline browser wwwoffle has the same problem. I wrote a patch for wwwoffle,
which makes saves "%3a" in URL pathes, instead of rewriting it to the colon (":"). 

I'm not familiar with apache2's mod_proxy* code. But probably the idea of saving
"%3a" also helps to fix the problem in apache2.