if i am playin somebody at 501 darts and they are left with double 5.with there 1st dart they hit double 5 but throw there other 2 darts because they didnt think the 1st was in and ref didnt tell them and the last dart lands in single 5 do they win or leave 5 to score
If you throw the other 2 darts from behind the "ockie" and a dart as you say lands in the single 5, then you have bust as you have scored 15. So you still have to get a double 5.
If you throw the other 2 darts behind the "ockie" and one of the remaining darts score in your case a single 5 then you have deemed to score 15 and therefore busted. So you still have to get a double 5. It is the players responsibilty to ask the ref his score and not the other way round. So in case it is the players fault for not asking the ref if the dart is in or not.