iOS 8.0 introduces the layoutMargins property on cells AND table views. This property isn't available on iOS 7.0 so you need to make sure you check before assigning it! The easy fix is to subclass your cell and override the layout margins property as suggested by @user3570727. However you will lose any system behavior like inheriting margins from the Safe Area so I do not recommend the below solut
![iOS 8 UITableView separator inset 0 not working](https://cdn-ak-scissors.b.st-hatena.com/image/square/98d6f053a97a87156775f60757c60865d0f2c47d/height=288;version=1;width=512/https%3A%2F%2Fcdn.sstatic.net%2FSites%2Fstackoverflow%2FImg%2Fapple-touch-icon%402.png%3Fv%3D73d79a89bded)