If you're a heavy user of Outlook tasks, you're probably filtering incomplete tasks in the current view. Unfortunately, this also means that as soon as you mark a task complete, it disappears. If you decide later in the day that it wasn't actually done, you have to go dig through all of your previous tasks to mark it incomplete.
More importantly, you also lose the sense of accomplishment from seeing things crossed off!
My solution it to modify the filter to show only incomplete tasks and tasks completed today. To do this, you'll need to enable the QueryBuilder dialog.
You can also change the second condition to include tasks from the past 2 days, the past week, or any other interval you decide.