item frame in item frame sometimes vanish

what i expect to happen: items inside item frames to not vanish to nothing

what happen:randomly the item frame vanishs rather that turning in to a drop

1st get a few item frames then place one down and put one inside it
2nd and repeat
3rd randomly the item inside vanish

http://youtu.be/NGal3mY2Qyc
youtube video
give it a while it uploading