yes, you can unmark a completed task. To do so, enter the project in which you've completed it, click on the "Show completed tasks" link at the bottom of the task list and simply click on the check-box next to it to un-check it.
Note - you can only use this option if you haven't used the "Delete completed" link which permanently removes the task.
I tried to follow your directions, but I only see 1 task at the bottom for the project.
At the top right, I see 5 tasks completed. It is one of those. Why don't those 5 completed at the bottom? They are all a part of the same project and were completed today.
in the top-right corner, Todoist stores your full completion history across all projects and that includes checking off recurring tasks (each occurrence is added as +1 completion) while the list at the bottom shows actually finished tasks.
Please note that if you've checked off a sub-task, it will stay on the task list unless you hold the SHIFT key while checking it off.
Try to check all your projects to find this task. You can also see in which project it has been on the "# Completed" history in the upper-right corner (simply click on this link and you'll see all details).
Oh, it's a recurring task that I marked complete. Is there a way that I can undo this? I accidentally clicked the checkbox.
It would be nice to be able to see the recurring tasks that I completed at the bottom "completed tasks" too.
if it's a recurring task, you'll need to change its due date in order to set it to a past date.
If you want to remove this event from your history, you can do so on the "# Completed" list. Simply hover over it and use the "X" icon to delete it (note - it will have no affect on your task list, it will only remove this occurrence from this list).