If it's an overly developed roast, or a roast with excessive defects like baking, scorching and unevenness (inside of bean darker than outside)
you will be extracting bitter solids which will be suspended in the crema, a percentage of which will
remain in the foam of the milk. If you leave the foam 'til last and then spoon it out, is it bitter?
Bitterness can also come from water that is too hot and a grind that is too fine for the extraction temp.
Bitterness is concentrated at the back of the palate, top of the throat.