Failure to chmod +x will usually result in 403 Forbidden. Internal Server Error is more likely to mean that the script didn't print valid HTTP headers followed by a newline before anything else on standard output. This can be due to the author just forgetting to do that, or it can be due to a logic flaw, syntax error, or other issue. (update: it can also be due to omitting the Content-type header, which is mandatory, at least under apache.) Whatever it is, it definitely means you should look in the error log. I usually tail -f the error log, hit enter a couple of times, and then hit reload in the browser. If the site isn't too high-traffic, this makes it easy to see exactly which errors were coming from the script in question.