hole in my head

Brass freeze plugs do not bite well at the iron and they can blow out if not installed fully, or if shallow ones are used (which are the more common ones). I'd replace them all and make sure when you do the bores are clean, free of rust, silicone, oil, antifreeze, etc. I use emery cloth to clean, and do not go "in and out" with it, go "around" the bore in the direction of the drill that finished them. Then I use a very small skim coat of grey RTV, and make sure you press them in so the edge is deeper than the chamfer on the bore.