Nytmare
David Jose
For instance, the Rod of Reaving doesn't work against minions. Presumably, a character would be able to observe that the RoR works against some monsters, but not others. And with sufficient observation, he might also discover that the monsters the RoR doesn't work against are precisely those that always go down in one hit.
I always like to believe that it's in pretty much everyone's best interest to not make the mistake of treating a battlemat like it's a science laboratory. The balancing act of a magic item's game mechanics and the way they interact with a creature's role is a lot of metagaming to try to rationalize away as deductive reasoning.
In a completely different vein, aimed at people who dislike monster knowledge rolls, what were your feelings in previous editions where players who memorized monsters out of the books had an advantage over players who didn't?