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.
There was a five-month gap between episodes 12 and 13 of Gravity Falls — October 2012 to February 2013.
And now it's been a year, and only about one or two episodes air each month, except for May, where none aired.
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.
Planet Sheen has been a victim of this, it began in 2010 and the first (and last) season didn't finish airing until 2013.
Season 2 and 3 of "Sponge Bob Square Pants" 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 7 of its 26 episodes have aired so far.
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!