You post no code, but here's a guess:

If using CGI, check only one header is being printed.

If printing header manually, check blank line appears after it, ie print "Content-Type: text/html\n\n"; - if you got this wrong, your server may be trying to guess the content type since you didn't supply one as it expected, and ends up printing an extra header.

cLive ;-)

Please try to think about code to show us when asking these questions, otherwise we just make stabs like this :)