I've noticed a few times recently where a user has a task assigned to them, but they report that they can't see the task in their task list.
If I log in as the process administrator, I can see the assigned task, but when I click on the instance-step to pull up the form, I receive the error "Task Unavailable - This task has been reassigned and is no longer available."
If I re-assign this task to another user temporarily, then back to the original user, they're able to see it in their tasks list and approve it.
Forms 10.1 Update 2