I do a simple:
In the top of this file, I have:
#DO NOT EDIT THIS FILE - edit the master and reinstall.
And I see that it has opened up some temporary file under /tmp
I am using Gentoo stable, with "vi" as my editor. I have followed the guide at http://wiki.gentoo.org/wiki/Cron#Installation to install dcron.
It is easy to find material on similar issues, i.e. the "DO NOT EDIT THIS FILE" string, however the "solution" is to use "crontab -e" instead of manually editing the file. I am using "crontab -e", but it still opens this temporary file.