Contains spoilers for the Lost Mine of Phandelver.
In the 5e Starter Set adventure, you encounter
a CR 3 Wraith (named Mormesk) (LMOP, p. 59). Its Life Drain attack reads slightly differently than the CR 5 Wraith in the Monster Manual.
I'm not referring to how their stats are different to reflect their CRs. Both of their attacks refer to the effect of reducing the target's HP maximum on a failed Constitution Saving Throw, but while the normal Monster Manual wraith's Life Drain says (p. 302), “The target dies if this effect reduces its hit point maximum to 0,” the specific enemy's Life Drain specifically says “If this attack reduces the target's hit point maximum to 0, the target dies” (emphases both my own).
So you can see that these are different. In one case a creature dies if the attack brings it to 0 HP (even succeeding on the Constitution saving throw), while in the other case the creature would still be alive at 0 HP (albeit unconscious) if the attack brought them to 0 but they succeeding on the saving throw.
Is this to make the monster in the adventure different from the monster in the MM, possibly to give the campaign a little flair, or is an error, possibly an inline update between the campaign and the MM? Does the Life Drain attack kill automatically instead of knocking out, or is it only on a failed save?