DIAGNOSIS The cup is compromised. Cracks typically form from impact, over-tightening, or blending extremely hot liquids which stress the plastic.
FIX Do not use the cracked cup. A crack will propagate under pressure from blending and can lead to catastrophic failure, leaking hot or cold contents onto the base and user. You must replace the cup. The part is a 16oz Fit cup, often sold in pairs (model-specific). Order an official replacement to ensure proper fit and lock with the base.
IF THAT DIDN'T WORK If the crack is very minor and only on the exterior, it may not immediately leak, but the structural integrity is lost. Using it risks sudden breakage. There is no repair for a cracked polycarbonate blending cup.
HEADS UP Never blend boiling liquids. Allow hot ingredients to cool slightly. Avoid dropping the cup or knocking it against hard surfaces. Hand-tighten only; using excessive force to lock the cup onto the base is a common cause of hairline cracks.