Arthur aired its first eleven seasons pretty regularly. However, when Season 12 premiered, it aired only the first 5 episodes of a 10 episode season during one week in fall 2008. The final five episodes were then burned off during one week in spring 2009. This pattern has continued until Season 18, which starting airing its episodes in the same pattern, however, it aired only the first four episodes of the season in fall 2014, holding the fifth until January 2015. The final five episodes were not aired in spring 2015. Episodes 7 and 9 were aired in June 2015 along with episodes 5 and 8 of the next season, while two episodes of Season 18 are scheduled to air in September 2015 with one STILL without an airdate.
Ankama, the producers of Wakfu have serious deadline issues. Firstly, the show was meant to be the The Cartoon Of The Game, only the game itself had been in closed beta before the cartoon aired. Secondly, the first season aired with significant gaps between episodes, sometimes as long as three months, because Ankama couldn't get it ready on time. Their deadline issues seem to be motivated by perfectionism rather than laziness, given the consistent quality when they do release something.
Young Justice aired its first 9 episodes between January and March 2011. Episode ten aired in September, followed by a regular schedule until episode 18 in November 2011. Episode 19 didn't air until March 3, 2012. Bizarrely, after the first season finished airing a few weeks later on April 21 (about 15 months after the season premiere), the show immediately proceeded with the second season premiere the following week, before another hiatus, between June 9, 2012, and September 29, 2012....before ANOTHER unscheduled, unannounced hiatus two weeks later, delaying the show (and the entire DC Nation block it was a part of) until January 5, 2013.
Recess went through a bit of schedule slip during its third and fourth seasons, with long gaps in between episode airings. This was mainly due to the show's staff working on Recess: School's Out.
Gravity Falls originally had a standard schedule of a new episode every week. Following an expected mid-season hiatus, all consecutive episodes have followed a schedule of every two-to-three weeks.
As of season 2, newer episodes of The Amazing World of Gumball have been taking very long periods of time to air. This include numerous hiatuses such as a five-month gap between episodes 55 and 56 (February 2013 to June 2013) and another between episodes 112 and 113 (March 2015 to July 2015).
Phineas and Ferb was subjected to heavy schedule slip, ultimately resulting in four seasons being stretched across eight years.
Much like Gravity Falls, Steven Universe had a normal schedule until a mid-season hiatus resulted in new episodes being aired between two to three months.
There are the Stevenbombs, however, in which new episodes air all within the same week. While they are popular and beloved by fans, they still take a long time to come.
All Grown Up!: The fourth season took two years to air. Not just that, but there was 13 months between the airing of that season's premiere and the next episode, and a year between the second and third episode of that season. As well as a 9 month gap between the seventh and eight episodes of the fifth (and final) season.
While true reasons for John Kricfalusi's firing from the production of The Ren & Stimpy Show remain vague, there are rumours going around that he would purposefully slow down the process of making episodes, so the censors would have less time to go over them. This is also supported by the fact that the Adult Party Cartoon spinoff was cancelled by Spike TV after a month of airtime because he only managed to produce 3 out of 9 episodes on time.
It probably also has a lot to do with the "Adult Party Cartoon" being a complete bomb.
For what it's worth, Ren and Stimpy's first season ended with only 6 episodes, while the first seasons of Doug and Rugrats before it and Rocko's Modern Life after ended with the standard 13.
Parodied by The Simpsons (in one of several digs the show made at its then-rival) when one of the nominees for "Outstanding Writing in a Cartoon Series" is the season premiere of Ren and Stimpy. The clip is a title card that reads, "Clip Not Done Yet."
The Fairly OddParents was the victim of this. 2011 was supposed to be a year long celebration of its tenth anniversary, with "specials every month and new episodes". By April, nothing of the sort had happened. In addition, FOP was very rarely aired in 2010 and a number of episodes from the seventh season took two years to be aired despite the fact that the season premiere for the eighth season has been aired. It wasn't until 2013 that a new season finally began.
In America, the third season of Danny Phantom had a very slow schedule. An episode from the middle of the season was heavily promoted, and aired in late 2006. The episode, titled Urban Jungle, included references to and elements from things that happened earlier in the season, which left viewers largely confused. The rest of the season didn't air until Summer 2007, one year after it aired in Europe and other countries.
In a similar vein to Danny Phantom, the final season of Avatar: The Last Airbender aired a lot later in America than in Europe and other countries, so all of the plot twists, reveals, etc. were all spoiled on the internet long before the season aired in America. Ouch.
The 2012 incarnation of TMNT has run into this a few times. The most notable example would probably be Parasitica, which was said to air a week after "Pulverizer Returns!", but then jumped to June 1st, before changing into June 29th. It finally aired the next month on the 20th.
Season 2 and 3 of "SpongeBob SquarePants" experienced this. While it only took 9 months for season 1 to air its 20 episodes, it took season 2 two years and nine months, and season 3 three years, to air the same amount of episodes. Some season 3 episodes have aired while season 2 was still in progress. Seasons 4, 5, 6, and 7 took roughly 2 years to air 20/26 episodes. Season 8 was a little better where it only took 1 year and 8 months to air its 26 episodes, but Season 9 takes the cake. Season 9 premiered July 2012, and only 14 of its 26 episodes are planned to air as of September 2015.
ChalkZone had a pretty slow schedule. The first episode premiered on December 31, 1999, with the rest of season one to air through 2000. However, Nickelodeon decided to hold the show off until March 22, 2002. The season only had six episodes and was plagued by constant repeats (the season ended in May of the same year), which even led a few fans to think Nick cancelled it after six episodes. Finally in May 2003, the second season finally premiered, even though it was produced a year earlier (though because of the first season being delayed for so long, THAT season ended up airing in Spring 2002 instead. No episodes were produced in 2001 due to the show's staff unsure whether or not the show was even going to air). While it was also a short season (eight episodes; the average Nicktoon season is roughly thirteen episodes), the episode airdates were spaced farther than season one to allow less time for the third season to air. By the time season four aired, Nick's interest for the show had waned and cancelled the show right in the middle of the season in June 2005 due to low viewership note which many have pointed out to be Nick's own fault for putting the show in only three timeslots a week either when the target audience was just going to bed or weekend mornings running up against more popular shows, such as Cartoon Network's The Powerpuff Girls. However, Nick decided to air the remainder of the season...three years later. The last episode to air on the normal schedule was "The Crush/Gift of Good Intentions/Snapshots II: Wild ChalkZone!/Go Pop" on June 22, 2005. The next episode ("The Quicksand Man/Vampire Cannibals of New York/Killer Breath/Time to Go Home" ) wouldn't air until June 11, 2008 with no advertisement and in the wee hours of the morning. To make matters worse; by the time the rest of the season aired, the original target audience who saw the show when it premiered would've been out of the demographic by then.
The South Park episode "Goth Kids 3: Dawn of the Posers" was supposed to air on October 16, 2013, but a power outage at South Park Studios the night before delayed production and the show missed its deadline for the first time. The episode ended up airing the following week.
The (originally-intended) Grand Finale of Kim Possible aired on April 8, 2005, actually early in comparison to the rest of the third season, as there were still five more episodes. Four of them were released over the next few months, but the fifth, "And the Molerat Will Be CGI", was rather oddly held back until June 10, 2006, fourteen months after So the Drama - by which point, work on the Postscript Season had begun!