if you accidentally check off a task, you can click on the "Get completed tasks" link at the bottom of the particular project and un-check it to put it back on the list.
In case of an accidental deletion of a task, the undo option will pop up at the top of the screen.
In case of deleting a whole project, there is no undo option, however, to prevent deleting it accidentally, we've put a warning message asking you to confirm the deletion which lists the names of all projects that will be deleted in the process (incl. sub-projects).
How does this work with recurring tasks? I cannot find an option to undo a click for these. I accidently clicked the November task thinking it was my October task.
unfortunately, it's not possible to un-complete a recurring task. You can of course remove this completion even in your history (by clicking on the "# Completed" link in the upper-right corner), but in order to change the task's date, you'll need to edit it's formula.
To do so, you can simply reenter the same due date formula, but add a starting date to it. For example, if the original due date is:
every last day
and the task has been mistakenly completed, you can change the date to:
every last day starting 30. Sep 2012
If you add the full starting date including the year, a task will be scheduled for the past and this way you can roll back a recurring task.