For example, we have the following chain affecting the same coins:
A → B → C → D → E
С and D are withheld. D is a defect. The operator tries to withdraw E.
The operator may be challenged by the following way:
Challenger commits B and requests to prove chain continuity. Operator challenges him by withheld C. As we can see in Challenge request part, challenger have second try and commit output of C. The operator cannot challenge C by defect D and finally his exit E is rejected.