One thing to remember is that your terminal session is done as a user who has logged in. Thus, your environment variables, default directory, etc, are set by /etc/profile, your own .profile, and so on. (If you use Korn shell; YMMV depending on your default shell.) crontab does not initialize this way, so if you need to use those things, you must explicitly run them.

This bites more than a few people, particularly those who are new to crontab.