Dawson Casting: Eight-year-old Mac is voiced Sean Marquette, who was about fifteen or sixteen when the show began. He was not replaced after he had gone through puberty. Instead, he had his voice digitally pitched up. It is obvious if one compares episodes from Season 1 to episodes afterward.
Fake Brit: Kansas native Tom Kane as Mr Herriman, who has a British accent even if he probably isn't actually British. He mostly pulls it off, but certain pronunciations may give the game away.
Ivan and Uncle Pockets, two one-off imaginary friends, are Pleakley and Gantu, respectively.
Keep Circulating the Tapes: Seasons 1 and 2 have been released on DVD, though both have been out of print for years (with sets of season 2 going for as much as $70.00! ). Luckily the entire series is available on iTunes and the Playstation Network.
As of November 2014, the third season is now available on DVD- as a Warner Brothers burn-on-demand exclusive.