Keep Circulating the Tapes: Various DVD compilations were released in the mid-2000s which are now out of print. Shout! Factory released the first 24 episodes before losing the rights to the series. Mill Creek Entertainment released the entire first season on DVD, one half in 2012 and the other half in 2013. This applies to the second season which has still yet to be released.
Off Model: Sometimes, Wordsworth's skates would disappear from his feet in between shots; it's especially noticeable at the start of the episode "Junk Food". In Season 2, Hector's ears would sometimes be colored beige (they're normally black), and then there's Cleo where sometimes her entire being was drawn horribly, such as in the episode "Hector's Takeover".
Talking to Himself: Riff Raff and Wordsworth were both voiced by Stan Jones, and then there's Mungo and Leroy, both voiced by Ted Ziegler.
Urban Legend of Zelda: Often times, someone will claim, or swear they saw an episode where Heathcliff and Riff Raff actually meet, but the episode only aired once for whatever reason. No such episode was ever made; the only time the two characters interacted was during the end credits.