It occurred to me that it might just be capillary action with the liquid flowing along the V-shaped channel formed by the table and the edge of the mug. Some googling did turn up the fact that ring-shaped coffee stains from single drops are caused by capillary flow: as the drop evaporates, it draws water from the inside.

But the instant ring from the mug? Either it’s something else, or it’s so obvious no one has throught it worth writing about.

Edit: *sigh* Read first, then post. I was just reminded that capillary action specifically refers to fluid moving against gravity. Any other thoughts?