Mobile Safari opens tab with window.scrollY set to large negative number, only Smart Banner visible
This appears to be a bug in mobile Safari. I was able to replicate this bug using a demo iOS app. Full details are here: https://github.com/than/Mobile-Safari-Smart-Banner-Viewport-Bug

Video of the bug in action: https://www.youtube.com/watch?v=IEILZ9uyH4g