“Under certain conditions, it takes a shorter time to cool a hot system than to cool the same system initiated at a lower temperature. This phenomenon—the “Mpemba effect”—was first observed in water and has recently been reported in other systems. Whereas several detail-dependent explanations were suggested for some of these observations, no common underlying mechanism is known.”
Source: Nonequilibrium thermodynamics of the Markovian Mpemba effect and its inverse PNAS, vol. 114 no. 20, pp 5083–5088, May 2107
There are various theories regarding an explanation of the effect, though none is yet universally accepted.
Further, there is still some debate around whether the effect actually exists, or is simply due to measurement anomalies.
Further info Wikipedia