[stillsut]

The interview's pressure makes finding these relatively easy tricks difficult. While in the flow of programming, or discussing the problem with colleagues, you'd probably knock this one out in a few seconds, the power difference in the interview does bad things to the visual reasoning part of your brain.

A good outcome I'd look for is the candidate recognizes that loop and mark will not always produce an exact number of mines.

So the ideal interview question I think gives the candidate is a simple task with lots of pitfalls that arise in the naive implementation. The goal is for the candidate to enumerate the pitfalls, and also develop an abstract representation of the problem. Leave the solution / most efficient algorithm out of it.