Pull down the list to reveal the “secret bar” that normally hides under the navigation bar. Tap the View button, then choose All. Uncheck the task you erroneously tapped, then use the View button to return your preferred view mode.
You can also shake the phone to undo an action, as long as the setting is turned on. To test it, create a new action, check it to mark it complete, then shake the phone sideways/diagonally. You should get a prompt asking you to confirm you want to undo the completion.
You can shake to undo multiple times – I just shook it a second time and it prompted me to either undo creating the test action or to redo marking it complete. I don’t know how far back this undo feature goes but it is kind of cool.