This has happened to me various times but its kinda fuzzy as I can't find anything official on it.
Lets say you tank a turret with a teammate so you think you're gonna kill the turret in time before you die but then you realize you won't make it in time so you decide to run away or even flash out if the timing of the game is crucial and you really don't want to die. You are only one hit and right before you flash the turret fires a missile which of course follows you to wherever you flashed. But before the missile lands on you and kills you, your teammate manages to kill the turret and when the missile hits you it makes no damage.
Now the same of course has happened to me with champion skills. For example I've killed a leblanc but she had already thrown her Q on me and it killed me even though it landed after she died. Or a nunu snowball or any other locked spell with a short delay.
I'm pretty certain you can't get damage from turrets if they get destroyed in the meantime but as I said I can't find anything official. Can someone confirm this or point to a link stating it? I've always wondered :)
EDIT: I'll use a picture to make it even more clear. If the turret in the picture was destroyed at that exact moment blitzcrank would take no damage from the purple missile heading towards him :)