I would argue that on a math midterm it's entirely reasonable to show a problem they've never seen before and test whether they've made the connection between that problem and the problems they've seen before. We did that all the time in upper division Physics.
A problem they've never seen before, of course. A problem that requires a solving strategy or tool they've never seen before (above and beyond synthesis of multiple things they have seen before) is another matter entirely.
It's like the difference between teaching kids rate problems and then putting ones with negative values or nested rates on a test versus giving them a continuous compound interest problem and expecting them to derive e, because it is fundamentally about rates of change, isn't it?
