How about the baby in "COLLECTIVE", from VOYAGER? Never referenced again in the series. But most importantly, not again after it was beamed to Sick Bay. Did it live? Die? Taken back to its home species? Seven had files on all 4 kids, but the baby was totally forgotten. And since the baby was important in a scene on the cube, and part of the plot of how their First couldn't take care of them or keep control... I call that a plot hole.