Textured milk, or am I crazy?

Here’s the situation: In a serve-yourself setting, I poured some milk from a carafe into some coffee. It made those familiar curdled-milk doilies. I smelled the milk. It smelled fine. I poured a little into a cup and tasted it. It tasted fine, except for the odd texture (i.e., curdled). I asked a passing wait-person, who expressed bewilderment and kept moving.

Is it possible for milk to be curdled but not sour? If so, what causes that condition?

Thanks!

Lemon juice can curdle milk. Is it possible there was a little lemon in your coffee?

Milk can also curdle under high heat.