操作をまちがえました!元に戻せますか? - Remember The Milkのヘルプ

If you make a mistake, don't panic! Most actions on Remember The Milk an be 'undone' easily. Once you perform an action, you'll see an orange box at the top of the page. It will prompt you to undo your last action.

If you click on the 'More Actions' drop-down, you'll also see an 'Undo Last Action' option. Choosing this option multiple times will undo multiple actions.