Tip: If you have a keyed cable-lock (or any kind of lock that doesn't need a key to lock), carry your lock around LOCKED, not UNLOCKED. If you're out and forgot your key, you might accidentally lock your bike up and strand yourself.

OR if your bike is a billion times more valuable than your time, DO carry your lock around unlocked, so even if you forget your key, at least you can lock it up if you have to leave it somewhere, and come back later with the key to fetch it.

Either way, think about it.