Unexplained CGI error

I had a very nice little application I was working on. I selected options from an html page and then after selection I called a PHP script. It worked just fine, but now when I use the same code unchanged, it gives me the error you see below and nothing more. This has happened to me twice before. What gives? There must be something I'm doing, but I can't figure what it is. I can't be the only person who's gotten this. Please help! Thanks, CGI Error The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are: