Why did the milk curdle in my coffee mug?

[Colophon]

@Really_Not_All_That_Bright

Sometimes milk droplets will catch in or around the spout of the jug, congeal, and form a very small crust. You may have loosened them up while pouring in your milk this morning. I’ve seen it a few times when making tea or coffee at home.

No, this was a *lot *of curdling. Like, when I poured it down the sink at work there were a good few spoonfuls of lumpy bits.