What will be the toughest interview (technical) question for an Electrical Engineer?

There is only a toughest (technical) question for an experienced Electrical Engineer, question that an Interviewer asked which is related with Engineering field but not related with your Work Experience. For example, if you have expertise in Control Systems and Interviewer ask you about Power Generation, it will be toughest question. OR

A question that you have expertise in the field of a single Vendor but Interviewer asked you about the same thing of another Vendor. For example, if you have expertise of Siemens PLC and Interviewer asked about ABB PLC, it will be tough for you since basics are same but as an Electrical Engineer you have to study about there software, communication protocol etc.

The toughest interview question might be about asking about salary of your last job because for electrical engineer freshers companies don't give high salaries. Not all companies but most of the companies don't pay high salaries for freshers.