"Professor Dumbledore!" Harry, in Snape's body, ran into Dumbledore's office, having recieved the password from McGonagall (who thought he was Snape).

"Yes, Professor Snape?"

"No, no, I'm Harry Potter!"

"The real Harry Potter came in earlier, claiming to be you, Severus.  Now you come up with this story?"

"I am Harry James Potter, son of-"

"Severus, everybody knows Harry's name and his parents' names."

"I told you- I'm not Severus.  I'm Harry.  Harry Potter."

"How do I know that?  How can I tell that?"

"Sir, Neville added too much wormwood to a potion, causing it to... er... not exactly... do what it was supposed to."

"Well, if you were Snape, I'd be ashamed that you knew that little about potions."

"So you believe that I'm Harry?"

"Let us call in whomever is in Mr. Potter's body, also," said Dumbledore thoughtfully.

*** 5 minutes later ***

Snape, in Harry's body, entered the office. "Yes?" he asked.

"Are you Severus?" asked Dumbledore.

"Yes."

"Why are you Severus?"

"That boy Longbottom ruined a potion by adding far too much wormwood, causing the bubbles in the potion to pop and the potion to become too thick, not to mention changing its chemical properties."

Dumbledore nodded. "I believe you now."

"Really?" Snape and Harry asked at once.

"Yes," said Dumbledore, "if what you say has happened to this potion, it should be wearing off within five minutes."

Harry smiled.  He'd be happy to be in his own body.