Why do people think Snape is Harry Potter's father isnt it obvious that James is Harry's father ??

What made the fans who didnt read the book think that way ?? Is there something in the movie DH-part 2 made them think Snape is Harry's father ?? When we came out of the theatre my sister was wide eyed and shocked I felt proud that I read the books and she didnt because she looked confused so I told her in the I-read-the-book-and-you-didnt tone "Al right Alice what is it that got you confused from the movie I can answer ANYTHING.." she turned to me and said: "One question: Is Snape Harry's REAL father" I just wanted to slap her when I heard that question and its not just her my friends asked me THE SAME THING. What made them think that way ?? What scene ??'._. I'm just really confused that they think that way.