I gave my students a take home exam over spring break. (This is normal where I teach) One of the questions was particulary difficult. It came down to a factor of three in the solution. That factor inexplicably appeared with no justification on many of their exams. I intend to have the students I suspect of cheating come to my office to solve the problem on the board. What would you do?
Not the same thing, but when I was proctoring an exam I saw someone very un-sneakily using their phone, so I quietly sat down next to them for the rest of the exam as a quiet threat (then of course let the prof know when they turned in their exam too).