What happens in Go if you "take back" a ko before making a ko threat?
In all official rules that I know this means instant forfeit of the game. Of course, in casual play, mistakes like this can be allowed to take back at the will of the players.

In tournaments I would strongly advise against making such a mistake as well as trying to ask for a take back. If the other player does not seem to allow a take back by his own active will, just resign and promise yourself to not make the same mistake again. Mental strength is part of the game, too, after all (just like time management, i.e. losing on time).

The reason players sometime forget to make a threat is often that after player A took the ko, player B takes some time to contemplate the threats, the board, counts the score, and finally simply recaptures the ko - the need to play a threat slipped out of their mind in the meantime.