Imagine my crontab
* * * * * /usr/bin/date > /tmp/foo
I am writing the date to /tmp/foo every minute
$ cat /tmp/foo Thu Jul 5 08:45:01 CEST 2012
Now I want to view my crontab in my EDITOR (vi).
$ crontab -e
I do not quit yet.
In the meantime, my colleague modify the crontab.
* * * * * /usr/bin/date > /tmp/bar
Later, I quit vi with :q!
O Surprise, the crontab is * * * * * /usr/bin/date > /tmp/foo again
According to the doc :
When you finish creating entries and exit the file, the crontab command
copies it into the /var/spool/cron/crontabs directory
Even if you did not make change, you overwrite the content of your crontab !
If you want to exit your crontab editor really without overwritting the crontab, you need to kill yourself.
CTRL-Z  + Stopped (SIGTSTP) crontab -e $ kill %2 $  + Stopped (SIGTTOU) crontab -e
Thanks to Colin comment, I realized I could not kill with kill, let’s kill with -9
$ kill -9 %2 $  + Killed crontab -e