lucy24 - 10:11 pm on Apr 18, 2012 (gmt 0)
Throw out the HTTP_HOST line. You don't need it. In fact, one job of your Redirect is to get everyone on the same page. Give the complete protocol and domain name, as in:
RewriteRule ^products/cat1/(.*)$ http://www.example.com/products/NEWCAT/cat1$1 [R=301,L]
As I typed out this line, a simple error jumped up and hit me in the face. I hope it's the same one you were running into. Hint: Move one parenthesis. In your rule as written, there are at least three equally valid solutions.