Strangely, despite being produced as part of this season, "Parents Day" was held over in the US for over a year and aired in 2000, as part of Season 5.
A deviation between production order and airing order caused a continuity error: "Rich Kid" was supposed to introduce Lorenzo as a new character, but by the time that episode was aired, he had already made appearances in "Gerald's Tonsils" and "Arnold's Room".
During its original run in 1998-1999, this season aired 20 half-hours. Out of those 20, 19 of them were episodes made for Season 3, and the remaining half-hour was episode 19, produced back in Season 1. It's unclear why that early episode was held over for so long.