Not at all surprised. In 2011 there was a small pond and residual ice up there in late June.
Mauna Kea is 13,800 feet above sea level. Hawaii is not on the equator (just under 20 degrees north). It is not surprising it would occasionally get cold up there--especially in the winter.
On youtube, there is a Richard Hammond Engineering Connections episode regarding the Keck Observatories, and there was snow.
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