Whenever I find myself having to change a MAC address, I end up using DEADBEEFCAFE.
I hope I never forget about changing them back and end up having to debug two different machines with the same MAC (which has actually happened to me in the wild, with two machines coming out of factory with the same MAC, talk about bad luck and shitty quality control).
I've seen duplicate MACs twice in the last few years, on two different lines of embedded/consumer electronics boards from two different factories. There was a kind of Abbot & Costello routine that went on the first time, when a Taiwanese colleague with limited English reported the problem to me.
I hope I never forget about changing them back and end up having to debug two different machines with the same MAC (which has actually happened to me in the wild, with two machines coming out of factory with the same MAC, talk about bad luck and shitty quality control).