.. a (D) action that requires a tapped minion as target fizzles, if the target minion manages to untap before the action resolves, e.g. when the minion untaps in reaction by using cards like Special Report or Black Sunrise.
Example: Great Kircher uses Nose of the Hound to rush a prince. The prince then uses a Second Tradition: Domain to untap. The action fizzles, since the prince is no longer a valid target for the Nose of the Hound action, but combat still occurs since the action was blocked.
The prince is (of course) still untapped after playing Second Tradition: Domain, but is tapped then for successfully blocking, even though the actual action has fizzled due to a missing target.
Similar examples apply to cards like Ambush or Blood of the Sandman.