Kassadin has a little buff icon above his ability bar to show when you have W active. When you die, you lose that icon and you lose the passive damage from W. When you activate W, you do not consume the charge when attacking an enemy minion, champion or monster.
1. Pick Kassadin and level W, and see the buff icon.
2. Attack some minions to see that it is working.
4. See that the buff is now gone and go attack some minions again to see you deal no passive damage, and the active is not consumed.
This happened with Classic and Harbringer Kassadin.
**EDIT:** This has been fixed with the latest patch (19th of December 2013).