And I would add:
Don't accept more than is absolutely necessary. Example: MSA's formmail.pl can be used as a spam relay because it accepts recipient as a CGI parameter. Don't do that.
Only accept the minimum number of parameters you need.
Don't trust anything from the client--period. I've seen cases where CGI scripts claim to be secure because they check REFERER, which is trivial to spoof.
Don't ever use a script that tells you to chmod a publically accessible file to 777 (Example: here).
Be highly suspicious of code that's been updated one or fewer times in the last several years. (Theory: No one was defending against DoS attacks in 1996.)
Not that I'm picking on anyone, of course. However, I have been helping a buddy run down various spammers that are playing on his system. Guess where the currently identified problems have come from?