Comment by hakunin

I don't think I ever try to verify the correctness of the code in code review. That seems hopeless to me. The original developer should be assumed to produce working code, which is covered with tests and maybe will go through QA (if you have that). Of course there are occasional bugs you can spot, but the main goal is to review the architectural approach and code clarity. You can also note if something isn't properly covered with tests. If you managed to make any security, performance, or edge case handing suggestions, that's a nice bonus.

Maybe we're in a very different context, but to me if you can't understand what the code does without a debugger, then you ask the author to make the code clearer.