Clipping Problem after following the "modeling nose cone" video

UPDATE: 

i should've waited till i finished the video haha, problem solved around the ~19th minute. 

(i knew that the knife operation was a bit odd :p )

cheers!

Hi, 

Around the 12th minute of the 4th video,  you used the knife tool to "quadify" the object as much as possible, i did follow along, deleted 3/4 of the object and then added the mirror modifier, and i ended up with this issue : 


even when i turn clipping on that still happens, then after adding the sub-surf modifier, the object looks like this : 

and the object has some weird effet in its center :


its more visible when i turn on AO in view port:


I re-watched that part 5 times trying to find out what i missed, but couldn't tell, it would really help, here is a link to the blender file if that may help.


thanks!