I think this is one of the more common mistakes that a seasoned sailor will make. I made it.
Last fall in Canada, I expected winter to follow appropriately. It did obediently. I knew that boats need to be winterized, as they say, to cut down on winter inactivity and winter activity.

The Shirley Anne's motor was fully purged of fuel and drained of any water. The water lines inside the boat still had water in them and the only way I could think of to purge those lines was to blow air through them. I had a 1/20th horsepower air compressor to do that with. No, not a machine; me. I ridiculously put my mouth over the line and using just my lungs, I blew air through the lines until I heard the hiss.

Well, it didn't work. I'm not sure WHERE the water came from, but there was water in the sink faucet. And, as the title of this post indicates, when the water froze, it expanded enough to rip solid metal apart. The photo shows the result.