I think that you are all missing the point if we were winning 1 - 0 at half time, the opponents completely being out played a mistake in the 85th minute evens it up and gives the other team renewed impetus and subsequently score another goal in time added on to win. So instead of getting 3 points as seemed most likely prior to the mistake, when well on top we finish up losing so no points. So in effect that mistake has cost us 3 points. It us arguable point that 90 mins we had 1 point so cost us 1 point but without the mistake would probably have got 3.
No, the only way that goalkeeper error has cost three points is if you want to criticise the goal keeper.
The subsequent goal isn't the result of the first error, it's either a second error (by any number of players/officials), or is the result of good play from the opponents, or luck.