Patrick and Mermaid Man are voiced by Ikuko Tani (an old lady) in Japanese; Plankton, however, is voiced by Chie Matsuraa, a young woman who doesn't even try to match Plankton's English voice.
Zig-zagged with Mermaid Man in Japan; he has been voiced by Ikuko Tani (Patrick), but also by Rokuro Naya (Squidward) and Keijin Okuda (Mr. Krabs).
The Danza: In "Mermaid Man Begins", the real names of Mermaid Man and Barnacle Boy are revealed to be Ernie and Tim. Mermaid Man and Barnacle Boy are voiced by Ernest Borgnine and Tim Conway, respectively.
Development Gag: Mr. Lawrence originally auditioned for the role of Spongebob, using the same voice he'd eventually use for Plankton. That deep, heroic voice Spongebob occasionally dips into? That's Tom Kenny imitating Lawrence's original voice for his own amusement.
Enforced Method Acting: Tom Kenny actually was very sick when recording Spongie's sniffly, sneezy dialogue in "Suds."
Executive Meddling: It has saved the show from two series finales. The movie was intended to be a Grand Finale for the series, but the possibility was later pushed to the "Spongebob's Last Stand" special. The show was continued since it was very close to dethroning Rugrats as the longest running Nicktoon. It eventually did bypass the series when it aired episode 173.
Irony as She Is Cast: David Bowie as Lord Royal Highness in Atlantis SquarePantis. He's the only character that doesn't have his own musical number in that episode.
Ever wonder why Mr. Krabs sounds so weird when he sings in "Welcome to the Chum Bucket"? Yes, that's Dee Bradley Baker singing, not Clancy Brown though he does his own singing in all other episodes.
Though SpongeBob often sings in his normal voice, on special occasions he shows off a fantastic singing voice that sounds nothing like him, with him being voiced by a different guest singer every time.
A few old episodes of the show seem to have recycled plots from other Nicktoons. "Toy Store of Doom", for example, has essentially the same plot as the Rugrats episode "Toy Palace" (they get locked in a closed toy store and are afraid the toys will attack them), while "Banned in Bikini Bottom" (Krabby Patties are outlawed and Mr. Krabs starts selling them at SpongeBob's house secretly) is similar to the CatDog episode "Just Say CatDog Sent You".
"Picture Day" has a recycled script from an episode of a Disney show: the Recess episode "One Stayed Clean". "Big Pink Loser" is also extremely similar to "Copycat Kid".
"Sailor Mouth" is very similar to "Bleeped", but airs much more frequently.
"Squid Defense" is very similar to the Hey Arnold! episode "Mugged".
"Face Freeze" is similar to season 1 episode "Hooky", wherein Spongebob and Patrick are tempted to do something Mr. Krabs told them not to.
Screwed by the Network: On their Nicktoons channel, as it only airs when most people are asleep. You were probably confused when you saw this trope on this page if you don't get Nicktoons. (That's probably because they don't need to air it on Nicktoons.)
Spoiler: The Dish Network description for "Doing Time" is "Ms. Puff imagines going to jail for SpongeBob," which spoils the ending.
Unintentional Period Piece: The pre-movie episodes, especially the first season which makes references to the pre-cell phone era. Many early episodes also reference the Krusty Krab being closed on Sundays at a time where many American businesses were also doing the same thing.
SpongeBob's spot in the Nicktoons roster was originally meant for another cartoon, one that never happened due to the creator leaving Nickelodeon over office politics. That cartoon? Sniz and Fondue of KaBlam!.
Early production sketches indicate the cast was originally human.
Spongebob was originally designed to resemble an actual sea sponge◊, but Stephen Hillenberg couldn't do an appealing caricature of one. He went with a synthetic sink sponge instead to give the character a "squeaky clean nerd" appearance, as well as to imply that he's "a square peg trying to fit into a round hole."
The creator also wanted to call him Sponge Boy until he learned the name was trademarked. This appears in a Mythology Gag where Mr. Krabs says "Sponge boy, me Bob!"
The name of the Krabby Patty was originally going to be the Barnacle Burger.
The original draft of "Something Smells" had Spongebob gaining his rancid breath from onion-flavored ice cream, but the writers decided that having it be self-inflicted was funnier.
Barnacle Boy was originally a character called "Barnacle Bill", a salty old sailor who's body is a piece of log with his head sticking out, similar to Seamus from Family Guy. While he never appears in the show, he's the titular character two-part comic "The Ballad of Barnacle Bill" in the Spongebob comic.
In terms of the series itself Word of God also states that in spite of the series continuing to this day, the original team members and the Hillenburg himself say that the first theatrical movie is the end of Spongebob's story.
Writer Revolt: According to Mr. Lawrence, the show was at least partially conceived out of Stephen Hillenberg's frustration over having several gags and stories he'd come up with during his tenure on Rocko's Modern Life get vetoed.
Write What You Know: In addition to his history as a marine biologist influencing the show's universe, Stephen Hillenburg based the Krusty Krab on an actual nautical-themed burger joint he worked at as a teenager, with his "old sea dog" boss serving as the inspiration of Mr. Krabs. The call-and-response theme song was based on one of the restaurant's gimmicks where the wait staff dressed up as sailors and lead the children in sea shanties, usually beginning with "Are you ready kids!" to which the children were expected to respond "Aye aye, captain!"