What defect is indicated by a dark "halo" at the edge of an espresso cup?

A dark "halo" at the edge of an espresso cup typically indicates that the espresso has undergone over extraction. When coffee is over-extracted, it means that too much of the coffee's soluble compounds have been drawn out during the brewing process, leading to undesirable flavors and coloration. This can result in a bitter taste that is often associated with dark or oily appearances, such as the 'halo' seen around the rim. The presence of this halo suggests that the extraction process continued beyond the optimal point, negatively impacting the overall quality of the espresso shot.

In contrast, under extraction would generally manifest as a light color and sour flavors, while perfect extraction would yield a balanced flavor profile without any undesirable visual defects. Excessive frothing relates more to the milk preparation process rather than the extraction characteristics of the coffee itself.