In the anime episode Kiba's Long Day (Season 4, Episode 184) there's a premise that a outsider had infiltrated the Village Hidden in the Leaves and used an infectious agent before committing suicide. Naruto has heard about Kiba "and the others" having dealt with this previous incident (and seems petulant about missing out). From the subtitles of the open:
Naruto: What do you mean by monitoring Akimaru?
Tsunade: You know about the incident where someone from outside infiltrated the village, right?
Naruto: Yeah, I know about that. But I heard Kiba and the others resolved that incident without any trouble.
Tsunade: That's right.
Naruto: Damn! That Kiba! I want a mission where I totally kick an enemy's butt.
Tsunade: To be exact, that's wrong.
Tsunade: Before Kiba and the others had the intruder in custody, he swallowed a pellet and killed himself.
Is this previous incident that Tsunade and Naruto refer to something that is portrayed earlier in the anime series? We'd seen some episodes where outsiders disguised themselves and entered the Village (and infiltration is a theme of later episodes as well). But "taking a pellet" to suicide doesn't match with anything I remember in the series to that point.