Ability to mark tasks as "canceled"
I would like to mark a task as "canceled" without deleting it, to indicate that it no longer needs to be done for some reason.
For an example of what I mean, one of my current tasks is "Email John on behalf of Sue." Sue no longer wants me to email John, so I'd like to cancel that task, but still keep a record of it.
Similarly, sometimes I simply screw up and miss a task's deadline. I don't want to mark it completed, because I didn't do it, but I also don't want to lose my record of it. A "canceled" option would be perfect for this.