Frequently, after GIR drinks something, he launches into an inexplicable coughing fit. What you hear are actual recorded real-life coughing fits of GIR's voice actor, the asthmatic 'Rikki Simons', who could not perform GIR's enthusiastic slurpy drinking noises without aggravating his breathing.
While the series did have exceptionally high ratings during its run, a majority of its viewers were in the teen and adult demographic despite the fact that it aired on a network targeted at children. Series creator, Jhonen Vasquez, stated that he intended to have this show appeal more to the teen demographic, which he seemingly accomplished.
On the Nickelodeon Animation Podcast, Richard Horvitz fondly recounted how Invader Zim's recording sessions got so spirited that they disrupted production of Dora the Explorer going on in the next room.
One of the many running gags in the show is the unnaturally frequent use of the word "Doom" and its derivatives. It's the most common word in the episodes' titles (not counting the "The", of course), and is also spoken by at least one character in most of the episodes themselves, especially in the first episode where it's uttered about 100 times by four different characters.
Four completed, unaired episodes were left hanging in the balance when Nickelodeon canceled the show. Those episodes are titled "Backseat Drivers from Beyond the Stars" (#2.1), "Mortos Der Soulstealer/ZIM Eats Waffles" (#2.2), "The Frycook What Came From All That Space" (#2.3), and "Vindicated!/The Voting of the Doomed" (#2.4). All four were included on Volume 3 of the Invader ZIM DVD release.