In Harry Potter: If only Purebloods get into Slytherin, How did Voldemort get in since his dad was a muggle?

Yeah, I know! I think is like a plot hole or something. JK obviously didn't think about some things. Or maybe she wasn't the only one writing the books (there were others helping)? Because she contradicted herself in this case. She also left blank the thing about the veil on the department of mysteries from the 5th book. (or maybe I'm missing something?) I would like an 8th book, wouldn't you?! :D