I like my PowerBook to be secure, so in the Security System Preferences panel, I require a password when coming back from sleep. I noticed a little problem though: assume I wake the machine, and after entering my password (but before the desktop is loaded), I close the lid, and the PowerBook goes to sleep. When I wake up the PowerBook the next time, it doesn't ask for a password, but shows my secrets to the world.
So don't close the lid and believe you're safe, unless you are very sure you are fully logged in. Especially when you move around a lot, and are used to just closing the notebook and stuffing it in you bag.
[robg adds: This hint was marked 10.4 only; I couldn't replicate it on 10.5, at least on my desktop machine.]

