The Fairly Sponge was a 2004 Nickelodeon crossover special between the series "Spongebob Squarepants" and "The Fairly Oddparents."
The crossover was cancelled when it was in final production, likely because Hillenburg denied Nickelodeon from using his character in any Nicktoons crossover specials (outside of cameo appearances and video games) as he wanted to keep his character self-contained. Because of this, the idea for this crossover was undone and instead became a crossover pairing the Fairly Odd Parents with Jimmy Neutron, which is titled "The Jimmy Timmy Power Hour" and also had two sequels.
On the other hand, a Season 5 schedule lists the unconfirmed crossover at the bottom. The schedule appears to look fake enough, due to it appearing as if it was made on Microsoft Paint, an image of the Fairly Odd Parents logo being copied/pasted, the text font and copyright format not being written the same as the kind Nickelodeon uses, and the resizing of the image. Judging by this, whether the crossover idea was officially created in the first place is unknown and is believed to be a fanfic idea created by a user by the name "Un robot xd," who stated in the comments that he [didn't create the idea and] actually found it in a 2004 blog about Nickelodeon, in a section called "Upcoming Episodes," but so far, no physical evidence of this has been put forward and the blog page has not been archived on the Wayback Machine.
As of June 8, 2013, Spongebob has made a cameo appearance in The Fairly Odd Parents episode "App Trap."