A "harry potter" is not a person, but in fact an action. If you go through any of the harry potter movies or books, you will notice that the main character - harry potter cannot go 5 minutes without doing something foolish, stupid or downright embarassing.
Therefore a "harry potter" is an action that is foolish, stupid, ignorant, embarassing, funny or reflective of something harry potter would do.
E.g. of in the movie, harry potter and the goblet of fire
*Cho chang and her friends on the ravenclawtable giggle and look towards harry. Harry notices this and smiles back, only to forget to his horror that he has milk in his mouth, it spills everywhere. Laughter commences.*
Mr Psst: Hey everybody i am going to walk forward
*mr Psst falls over*
Everyone else: Dude, you just did a harry potter.
See harry potter, funny, pwned, noob, n00b, idiot, epic movie, stupid, dumbass