Making tea - milk first or tea first
I had the chance to ask a tea scientist this very question about 25 years ago, and he said:


if you put the tea in first, over decades your cups will be more stained than if you put the milk in first
if you put the milk in first, you cannot add less milk on discovering that the tea is weak or there is less of it than you thought


He further reported that the Royal Family always put the tea in first. So, despite being raised milk-first, I am a tea-first person (the milkiness of my tea being more important than possible stains to my cups) and my mother tolerates this when she visits.

Killing bacteria in the milk is really not an issue now and probably never was; even if it were the temperature difference is not important between the two approaches. A little cold milk in a room temperature cup probably won't prevent thermal shock either: many people claim leaving the spoon in serves this purpose, but I think it's just to avoid putting the spoon down on the counter and leaving a dribble.