That's a good point and you are right. I have simply asked for the engineer to write the test in the past, but sometimes my curiosity just gets the better of me and I want to try it out myself. Other times I am not quite sure what is wrong but it just "feels" wrong so I end up throwing a couple unit tests at it to see if my intuition was correct.

Note that this is quite rare and is maybe a once in a 100 code review occurrence. Usually it's quite obvious that something is wrong and you can just point it out or start a discussion about it.