The name wouldn't be a grammatical error if it were referring to a grave that contained lovers, as opposed to the more obvious possible meaning of a grave that one/many lover(s) were in the possession of. For example: Rather than "Oh, that's her lover's grave" or "that's their lovers' grave," it would be "Well gee whiz! This can't be a jelly grave, there is no jelly within it. Instead it contains lovers and ergo it must be a lovers grave." ............... right?