"Tell me, Harry," Dumbledore said "How did Lucius Malfoy come to be at the bottom of a stairwell with no less than five pikes and lances protruding from his corpse?"
"He tripped, sir."
"When did he trip, Harry?"
"After I left your office to return Tom Riddle's diary, sir."
"I See, Harry." Dumbledore's Eyes Twinkled™. "And how did he trip?"
"He tripped over his cape and fell over the railing on the 7th floor balcony down the Hall from your office."
"I see, and how did the suits of armor come to fall on top of him?"
"They flew off of the balcony after Mr. Malfoy, I suspect he latched onto them with accidental magic."
"Harry, I was not aware of any suits of armor in the 7th floor hallway nor on the balcony," Dumbledore replied gravely.
"I think they were enchanted, sir," Harry replied, with utmost care to keep the laughter out of his voice and the smile off of his face.
"Enchanted, how?"
"I think they only appeared when no one was looking at them," Harry said seriously, which is not to be confused with siriusly. "I didn't see them until they flew over the railing after Mr. Malfoy."
"Ah," Dumbledore said. "That must be where Phinneas put the experimental stealth armor," he said with another Eye Twinkle™ and a nod. "Thank you, Harry. That will be all."
Harry carefully schooled his features into neutrality until he was well away from the headmaster's office.
"Wow, I guess Sirius was serious," he said to himself as he finally allowed himself to grin. "You really can get away with anything as long as you keep the first rule in mind."