Unit testing: I had candidate going really deep in the circuitry of the machine, testing each individual transistors. Most go to a higher level and test each individual functional component. Functional testing: Some candidate approach functional testing through the actual mechanics of the machine (buttons, display etc.) while some other approach it from a software angle, leaving aside the mechanics. The best tester covers both. White box/Black box: Again, your mileage may vary and you really see where your candidate is the most comfortable. Performance/reliability testing: How fast can I get my coffee? How many coffee can I deliver in rush hours, MTBF etc. You can have a lot of fun in this area. Usability: So much to do in the area but most candidate forget about it. Localization testing: Again, most candidate don’t think about this one. Security testing: Most candidate don’t go there but it’s always a good sign if they do. I had candidate forcing the door to get money, throwing water on it to see how far they can go until a power surge, sending 300v in the thing to understand how it would react etc.