Well, this is the first episode where Brock "identifies" Nurse Joy as an imposter (I said "identifies" in quotes because the way it was done implied that he only saw through the disguises to save face, as he didn't seem to even realize that it's a disguise, never mind the fact that the imposter in question is Jessie of Team Rocket).
Geez, if he were supposed to be the brightest of the group, why does he end up falling for the disguises most of the time, anyways? I mean, even Misty actually saw through some of them.