If you mark a task as complete that has child sub-tasks, all the children are also marked as complete.  If you want to make everything not complete again, you have to go to each sub-task and uncheck the box or right click and "mark as incomplete".  I have a few tasks with 10-12 child tasks, so each time I accidentally check the box when I'm trying to open up the children instead it is pretty annoying to have to uncheck every sub-task.

Reproducible: Always