iOS 8 Safari in Iphone, appending to autocompleted url
Backspace will return you to the point you were last typing.

Tap/hold will turn the autocomplete into 'real' text you can then edit - but you can't fill from halfway down the potential URL, it becomes a new full text string that you need to work with, as though you'd already typed it all.

So, for instance, typing app will turn into apple.com Tap/hold will 'freeze' that allowing you to then complete apple.com by typing /knowledgebase but if you wanted apple.net you would first have to delete the com before continuing.