Here is a link to a fascinating post on this subject from earlier this year at Skulls in the Stars. The effect has apparently been observed for millennia (the post contains a reference by Aristotle!), but the conditions under which it occurs and the scientific explanation for the Mpemba effect are still unclear.
Interested experimenters with room in their freezers (Dr. SkySkull mentions that his freezer is full to the brim, on account that he’s married … I’m pleased to report that I’m in the same boat) might like to explore this phenomenon.
(This post first appeared at my other (now deleted) blog, and was transferred to this blog on 21 January 2021.)