When they were originally casting for Ellen, the only one who fit the role at the end was Alison Fanelli, who hadn't acted before, but was cheerful (compared to the plain other girls who were auditioning for the role) and dressed a little "geekier" than the other auditionees at the time she auditioned.
Also, Danny Tamberelli. Often in the reunions, the creators have mentioned that when they were shooting the first short, they were incredibly worried of having a child buried up until the neck (it makes sense in context, little Pete was in a bet with his brother and Ellen about what would he do for a dollar). But at the end of it, he just thought that the whole experience was awesome. They realized that this kid was quite energetic and would accept every single weird thing they would think up for him.
Michael Maronna cut his hair before the end of the shooting of the final episode. His haircut was worked into the episode's plot.
Dawson Casting: Quite averted with the show since the actors continually grew up with the characters from the beginning in 1988 to 1996. Quite an straight example, however, would be some of the cameos. For example, pick Selma Blair (before being Selma Blair). She was 24 when playing a teenager in a Catholic School.
Edited for Syndication: Once Nickelodeon picked up the series and finally made it into half-hour episodes, the previous aired specials were re-edited into standard episodes to fill a full series of 13 episodes. They needed to add the Title Sequence, the "Hey Sandy" song, but also that meant that the episodes needed re-scoring with the standard cues. The most jarring example was "How We Spent Our Summer Vacation". Not needed to say, Katherine Dieckmann, the director of this particular episode, agrees that it destroyed part of the special.
Popularity Redo: The show was a series of advertisements for Nickelodeon before becoming a series. Many of the one-minute shorts were expanded or reworked into half-hour episodes throughout the show's run.
Showrunners McRobb and Viscardi would cast Damian Young (Bus Driver Stu) in their show The War Next Door, as well as the film Snow Day (which was proposed as a Pete & Pete movie originally, see below for more details). Bob Mittenthal, who wrote a couple of episodes, would also go on to co-create KaBlam! with McRobb and Viscardi. By a startling coincidence, Rick Gomez (Endless Mike) was cast in KaBlam! for one of their regular shorts, Sniz and Fondue— without any of the three knowing about it until production started.
Also, surprisingly, Scrubs. Several longtime members of Pete & Pete's production crew ended up working on the show, including P&P's director of photography Michael Spillman, who would direct over a dozen Scrubs episodes. David Martel, the actor who played Teddy on P&P, also had a couple of bit roles sprinkled throughout and when you consider how much of the show's style is eerily reminiscent of P&P (first-person narration with ending realization, absurdist humor and characters, emphasis on music, etc.), it's almost an adult Spiritual Successor.
Schedule Slip: Season 3 suffered badly from this when it was being aired. How can it be that half of a season would have been slipping away from late-1995 to late-1996 (the first episode of the season was aired on October 1st, 1995... the last episode of the series was originally aired on December 28th, 1996!)?
Nickelodeon messed up a lot with the show during Season 3. Viscardi and McRobb once replied in an interview that while it was hard to get them to pick it for a third season, as the series didn't exactly have strong ratings (compared to monsters like The Ren & Stimpy Show and The Secret World of Alex Mack), the guys at Nick loved the show and greenlit it. However, the discrepancy between the original airings of the mid season episodes must mean something (hopping from November 1995 to March 1996 and from April 1996 to November 1996)...
And then we have screwed by the parent company of the network. When Viacom shifted around some key executives at Nickelodeon, besides the separation of the friendly relationship of Viacom and CBS, it eventually resulted in several "Rewind" DVD line releases being postponed and eventually cancelled, leaving to the third and final season's supposed 2006 DVD release being scrapped. There goes the internal joke between the creators and cast that Viacom must have the DVD contents in a warehouse around New York or New Jersey. Everything was ready to be released (audio commentaries and some extras, the art of the DVD cover running around was the official one)... until the plug was pulled.
Sunday Midday Death Slot: The series had an unusual schedule. Around its peak, it was aired at Saturday night alongside other SNICK series. But it was eventually moved into 1 PM in a Sunday? It seems like the series ended in a non-existent schedule for the channel.
If the movie script would have been picked up, it would have been the first Nickelodeon movie produced, instead of Harriet the Spy, starring P&P alumna Michelle Trachtenberg.
Viscardi and McRobb pitched the script of the movie around the time the production of the final season was wrapping up (1995). The executives liked it, but after a time, it was obvious that they lost interest in it and shelved it, they were pretty sure the series could have been forgotten after a certain point.
There's also the story that when the script was pitched, there was a re-shuffle at the top at Viacom, causing the project to be shelved for four to six years.
By the time the script was picked up again, the original actors were clearly older than they were when the series ended and perhaps, involved and moved on to other projects note (Michael Maronna would have been 20-21, and looked almost the same, but the same could not be said for Danny Tamberelli, since he was 16-17 already, and most of the other kids had also changed noticeably). So, Nick executives asked for rewrites to change from the established characters to original ones, although it is somehow easy to see who would have been who. However, the spirit was still in there, since not only was the film's director a veteran director from the television series, but Iggy Pop and Chris Elliot also got cast in it.
Word of Saint Paul: As mentioned in the main page under the Indecipherable Lyrics trope example, there is still doubts that have launched several Wild Mass Guesses about a misinterpreted and misheard lyric of "Hey Sandy", since neither Viscardi nor McRobb have any idea of what the line really says, neither the DVDs subtitles are correct, and probably the only one to know is Mark Mulcahy himself. However, some years ago in an AMA done in Reddit, Danny Tamberelli claimed that the lyric could be "Can you settle a sure bet?", and has also sung that lyric when performing a cover of the song in one of the reunions made around 2011 and 2013. It would make sense, as it properly fits the song's rhyme scheme, but it's still his interpretation of it, not completely confirmed as the real lyric.