Say I hit my ball into some rough near the green, and after a bit of searching find a ball I assume is mine (though I fail to check) and hit it up onto the green.
What happens when I discover the ball isn't mine (stroke play):
Upon first encountering it on the green?
After putting it one or more times on the green?
After holing out?
After teeing off on the next hole?
(My understanding is that in a tournament situation at some point you are DQ'd, right? - fair enough - but how would you score it for your own purposes in casual play?)