Sherlock is sitting up when John reaches him, and he's looking rather well for someone who just fell down the stairs. Moving means no skeletal injury (though his neck needs to be checked), and there is no obvious bleeding. He is smiling though, which is odd.
"Hi, John," Sherlock says when John sits down next to him.
"How do you feel?" John asks, because Sherlock should at least be bitching about the staircase manufacturer's general incompetence.
"You have pretty hair," Sherlock says, still smiling.
"It's not funny," John says, and starts palpating Sherlock's head in case there are injuries his hair is hiding. He can't find any, but when he tries to remove his hands Sherlock grabs them and puts them back on his head. Bewildered, John starts stroking through the hair, and soon he has Sherlock lying across him, sprawled out on the pavement and pushing his head into John's hands.
"Sherlock, what's going on?" he asks, because Sherlock had never before given any sort of hint that he might like being, well, petted.
"I'm a cat," Sherlock says, and John moves his hands in shock, managing to rip some of Sherlock's hair out while he does it.
Managing to get Sherlock to stop clawing at him, and then getting him home, had not been easy, but John had managed through a combination of bribing him with more petting and promising cream when they got home. Once back in Baker Street Sherlock had refused to go inside, claiming he needed to investigate some fascinating smells, which had given John time to phone Mycroft in a panic. He had no idea what he had said, if he had even managed to explain that Sherlock now believed himself a cat, but Mycroft had promised to come immediately, so the direness of the situation had not gone unnoticed.
John did some quick research on the net while waiting for Mycroft, but he couldn't find anything that would make a grown man suddenly believe he was a cat. Well, nothing apart from several hallucinogenic substances, but he was almost sure Sherlock hadn't taken any of them.