Another thing to consider when having CGIs write to files is the physical location of those files.

I believe the safe path (at least in *NIX) is not to write CGIs to system-critical partitions, such as /tmp or /var (and probably some others) which, when full, can bring down a system.

To the Sys Admins out there: please correct me if I am wrong, but I do believe this to be a good practice; at least one I was taught long ago.