We know from recent probes that there is water ice at the poles of the Moon. New analysis of lunar samples brought back from Apollo indicates that water is, or was, many times more common in lunar rocks than anyone has guessed. We are not talking about a ready supply to support human habitation - far from it - but evidence of water in rocks from many locations is a big, big puzzle for scientists studying how our satellite was formed. It has long been assumed that the high temperatures of the impact even which formed the moon almost instantly boiled off any water "captured" from the bodies inolved.
There are several lessons here, including "don't get too comfortable with theories no matter how long they've been accepted" and "just because evidence has been studied for decades doesn't mean it's a waste of time to study it again."