There are several unused animations which can be found deep within the game's files. Most of the animations are unused attacks for Crash, nearly all of which have animations and are usable by modding the game's code. They were most likely removed due to time constraints.
There was once a character planned for the game called Capu Capu who was ultimately cut. He was originally the captain of the boat that took Crash to Iceberg Laboratory. He was cut as they had left it until the last minute to add him to the game, by which point finalizing the game had already begun.
It was intended for Fake Crash to appear as a cameo, stranded with Crunch on the small isle next to N.Sanity Island, before being scrapped for technical reasons.
According to a developer using the alias FakeNina, the line about Cortex almost calling Nina his daughter was added as a joke to make their relationship more mysterious. It's also hinted that she could very well be his daughter, and that only the future will tell.
The early form of Twinsanity, known as Crash Bandicoot Evolution, was a quest based game revolving around Crash trying his to help the "Gaudi" people. This would have usually resulted in him causing more problems than he solved.
The development team had plans for a sequel to Twinsanity, which would have involved Crash being transported into various TV shows via a device invented by Cortex. It never went past the brainstorming phase, due to the closure of Traveller's Tales Oxford.
Some of this game's music is based directly on classical themes. The music from the Doc Amok segment in Totem Hokum is based on Flight of the Bumblebee. The music from the worm chase in the same level is based on the Blue Danube Waltz. Finally, the music in the chase scene with Rusty Walrus is based on the first part of the Hebrides Overture.
A level called "Megalomaniac Mind Mania" was cut at a very early stage of development. The Psychetron would have transported Crash into Dr. Cortex's Brainovial Hyperbarium to explore Cortex's twisted mind and discover the connection Cortex has to the Evil Twins. Every enemy character was an ugly version of Dr. Cortex and would have driven around in tanks. The whole level was to be a musical number, sung by Dr. Cortex called 'Take a Walk Through My Brains'. Lyrics for the song were written, but ultimately after the level was cut, went unused.
The developer, Traveller's Tales Oxford Studios, was created specifically for the revival of the Crash Bandicoot series, with this being their first game.
A level called 'Gone A Bit Coco' that took place in Coco's mind was cut late in development because it was glitchy and incomplete. It can be seen in early gameplay videos, and the level music was used in the credits sequences.
The music of Crash Twinsanity was composed by an a capella group called Spiralmouth. This means that none of the soundtrack uses instruments, and was composed entirely with the human voice.
Originally, Coco's evil counterpart was going to appear in the game. However, she was removed for unknown reasons. She still remains in the deleted scenes for the game.
N. Trance was originally going to make an appearance in Crash Twinsanity in a cutscene in which Evil Crash eats his brain with a spoon "like an egg". The scene was scrapped because it was "a bit gross". The only trace of N.Trance left in Twinsanity is his concept art.
Before going to the 10th dimension, Dr. Cortex says "Come now, as we explore a new dimension! It should have been two new dimensions but we... ran out of time." This is a reference to the development period of 12 months, as the development team originally wanted to put another world into the game. However, due to time constraints, the world was scrapped.
What came to be of Crash Twinsanity bares little resemblance to the original design. Originally called "Crash Bandicoot: Evolution", the plot involved the Evil Twins and the Ants. They served as the antagonists and were traveling to different planets to steal materials to make their own planet. The game was changed due to the plot being too similar to "Ratchet and Clank".
There were suppose to be four playable characters, Crash, Coco, N.Cortex, and Nina. However Coco was removed as a playable character and made an NPC. Her game play was rumored to consist of her hacking into security systems. Although she is not playable in the game anymore, the back of the box still says there are "4 playable characters."