If the target creature is an illegal target by the time Lifecrafter's Gift tries to resolve, the spell doesn't resolve. No creatures receive +1/+1 counters.

If there is a +1/+1 counter on a noncreature permanent you control, such as a Vehicle that isn't crewed, it won't get another one from Lifecrafter's Gift.

The state-based action that removes matching +1/+1 and -1/-1 counters won't check until after Lifecrafter's Gift finishes resolving. It's possible put a +1/+1 counter on a creature with a -1/-1 counter on it and then give it a second +1/+1 counter.

Each +1/+1 counter put on the target creature is a separate event. Replacement effects may affect each of these counters being put on the creature.